Spring-loaded scale Jackscrew and crank Fabric strip Pounds Fixed base FIGURE 3-20. Fabric tensile tester.
determining whether a fabric covering has deteriorated to the point where re-covering is necessary.
Figure 3-20 illustrates a typical fabric tensile tester.
A sample of the undoped fabric to be tested is cut to exactly 1-1/2-in. wide and to a sufficient length (usually 6 in.) to allow insertion in the fabric tester. Usually, each edge of the strip is frayed 1/4 in., reducing the woven width to 1 in. The ends of the fabric strip are fastened securely in the clamps of the tester. As the crank of the tester is turned, the threaded jackscrew is backed out, thus gradually increasing the tension (pull) on the fabric against the resistance of the spring-loaded scale until the fabric strip breaks. The scale reading, taken at the moment the fabric strip breaks, indicates the strength of the fabric in pounds per inch.
Fabric specimens must be tensile tested in the undoped condition. Use acetone dope thinner or other appropriate thinning agents to remove the finishing materials from the test specimen.
STRENGTH CRITERIA FOR AIRCRAFT FABRIC Present minimum strength values for new aircraft fabric covering are provided in figure 3-1.
The maximum permissible deterioration for used aircraft fabric, based on a large number of tests, is 30%. Fabric that has less than 70% of the origi nally required tensile strength is not considered airworthy. Figure 3-1 contains the minimum tensile strength values for deteriorated fabric tested in the undoped condition.
Some light aircraft operators use the Grade A type fabric, but are only required to use intermediate grade fabric. In this case, the Grade A material is still considered airworthy, provided it has not deteriorated, as tested in the undoped condition, below 46 pounds, i.e., 70% of the originally required tensile strength value for new intermediate fabric.
DOPES AND DOPING To tighten fabric covering and to make it airtight and watertight, brush or spray the cloth with dope.
A tight fabric cover is essential to securing and holding the cross-sectional shape of the airfoil to the form given it by the ribs. This dope also protects the fabric from deterioration by weather or sunlight and, when polished, imparts a smooth surface to the fabric and reduces skin friction. Dopes must be applied under ideal conditions to obtain satisfactory and consistent results. A clean, fresh, dry atmosphere with a temperature above 70° F.
and a relative humidity below 60%, combined with good ventilation, is necessary in the dope room. The dope must be of the proper consistency and be applied uniformly over the entire surface.
Dopes will deteriorate seriously if stored in too warm a place for a long period. The temperature should not exceed 60° F. for long-time storage and must not exceed 80° F. for periods up to 4 months.
Precautions against fire should be taken wherever dope is stored or used because of its flammable nature. Dope and paint rooms that are not located in a separate building should be isolated from the rest of the building by metal partitions and fireproof doors.
As stated previously, the most desirable condition in a dope room is a temperature above 70° F. and a relative humidity below 60%. At lower tempera tures the dope will not flow freely without the addition of excessive thinners. The relative humidity can be lowered by raising the temperature if the dope shop is not equipped with humidity control.
To condition fabric surfaces to the desired temperature and moisture conditions, allow them to stand about 4 hrs. in the dope room after covering and prior to doping.
The number of coats of dope applied to a fabric surface depends on the finish desired. It is customary to apply two to four coats of clear dope, fol- 107
lowed by two coats of pigmented dope. Sufficient clear dope should be applied to increase the weight of the fabric by 2.25 to 2.50 oz./sq. yo. The cleardope film should weigh this amount after drying for 72 hrs. With fabric weighing 4 oz., the total weight of fabric and dope is approximately 9.5 oz./sq. yd.
Pigmented dopes must be applied over the clear dopes to protect the fabric from the sunlight. Sufficient pigment must be added to the dope to form an opaque surface. Pigmented dopes consist of the properly colored pigment added to the clear dope.
When an aluminum finish is desired, 1 gal, of the clear nitrocellulose dope is mixed with 12 oz. of aluminum powder and an equal additional amount of glycol sebacate plasticizer. Sufficient thinner is then added so that two coats of this dope will give a film weight of about 2 oz./sq. yd.
Panels should be doped in a horizontal position, whenever possible, to prevent the dope from running to the bottom of the panel. Hand brush the first coat of dope and work it uniformly into the fabric. A minimum of 30 min, under good atmos.
pheric conditions should be allowed for drying between coats. Surface tape and patches should be applied just prior to the second coat of dope. This second coat should also be brushed on as smoothly as possible. A third and fourth coat of clear dope can be applied by either brushing or spraying.
These coats of clear dope provide a taut and rigid surface to the fabric covering. If desired, this surface may be smoothed by lightly rubbing with number 280 or 320 wet or dry sandpaper, or a similar abrasive. When being rubbed, all surfaces should be electrically grounded to dissipate static electricity.
The doping is completed by spraying two or more coats of the properly colored pigmented dope on the surface.
Under certain unfavorable atmospheric conditions, a freshly doped surface will blush. Blushing is caused by the precipitation of cellulose ester, which is caused largely by a high rate of evaporation and/or high humidity. High temperatures or currents of air blowing over the work increase the evaporation rate and increase blushing tendencies.
Blushing seriously reduces the strength of the dope film and the necessary precautions should be taken to guard against blushing. When a doped surface blushes, it becomes dull in spots, or white in extreme cases.
The surface under the doped fabric must be protected to prevent the dope from "lifting" the paint on the surface. A common method is to apply dopeproof paint or zinc chromate primer over all parts of the surface that come in contact with doped fabric. Another excellent method is to cover this surface with aluminum foil 0.0005 in. thick. This foil is glued to the surface and prevents the penetration of dope. It is applied over the regular finish.
Other materials, such as a cellophane tape, have also been used successfully in place of aluminum foil.
DOPE MATERIALS Aircraft dope is any liquid applied to a fabric surface to produce tautness by shrinkage, to increase strength, to protect the fabric, to waterproof, and to make the fabric airtight. Aircraft dopes are also used extensively in the repair and rejuvenation of aircraft fabric surfaces.
Aircraft dope is technically a colloidal solution of cellulose acetate butyrate or cellulose nitrate. If nitric acid was used in the chemical manufacturing of the dope, it is known as cellulose nitrate dope. If acetic and butyric acids were used, the dope is known as cellulose acetate butyrate dope.
Cellulose-Nitrate Dope Nitrocellulose dope is a solution of nitrocellulose and a plasticizer, such as glycol sebacate, ethyl acetate, butyl acetate, or butyl alcohol, or toluene. The nitrocellulose base is made by treating cotton in nitric acid. The plasticizer aids in producing a flexible film. Both the plasticizer and the solvents are responsible for the tautening action of dope. Thinners such as benzol or ethyl alcohol are sometimes added to the dope to obtain the proper consistency.
These thinners evaporate with the volatile solvents.
Nitrate dope flows more freely and is more easily applied to fabric than butyrate dope. It burns read ily and rapidly and is difficult to extenguish, whereas butyrate dope burns slowly and is easily extinguished. The tautening (shrinking) effect of nitrate is not quite so great as that of butyrate, but it is sufficient to tighten the fabric the desired amount.
Cellulose-Acetate-Butyrate Dope This type of dope is composed of cellulose acetate butyrate and a plasticizer, triphenyl phosphate, which are nonvolatile when mixed with ethyl acetate, butyl acetate, diacetone alcohol or methyl ethyl ketone, all of which are volatile.
Butyrate dope has a greater tautening effect on fabric and is more fire resistant than nitrate dope.
The solvents of butyrate dope are more penetrating than those of nitrate dope, and butyrate dope can be applied successfully over dried nitrate dope on a fabric surface.
Both the cellulose nitrate and cellulose acetate butyrate dopes, without the addition of color pigments, are a clear, transparent solution. Both are used on aircraft fabric covering to shrink and tighten the fabric to a drum-like surface, to impregnate and fill the fabric-mesh, and to waterproof, airproof, strengthen, and preserve the fabric.
Pigments of the desired color may be added to the final two or three coats of dope applied to the fabric to attain the desired color and trim on the
ALUMINUM-PIGMENTED DOPES When at leat two or more coats of aluminumpigmented dope (brushed or sprayed) have been applied over the first two or three coats of clear dope after they have dried and have been sanded, a thin film of aluminum is formed over the fabric and the undercoats of clear dope. This aluminum film insulates the fabric from the sun's heat and reflects the heat and ultraviolet rays away from the fabric surfaces of the aircraft.
Aluminum-pigmented dopes may be purchased already mixed and ready for application by brush or spray. However, it is often more economical and desirable to mix the powdered aluminum into the clear dope in the shop.
The aluminum for mixing into the clear dope may be obtained in either the powdered form or the paste form. In the powdered form it is nothing more than finely ground (pulverized) aluminum metal, In the paste form the powdered aluminum metal has been mixed with an adhesive agent to form a putty-like paste.
Recommended mixing proportions are 1-1/2 lbs.
of aluminum powder to 5 gal. of clear dope, or 1-3/4 Ibs. of aluminum paste to 5 gal. of clear dope.
First, thoroughly mix and dissolve the powder or paste in a small amount of alcohol thinner and then add to the clear dope.
TEMPERATURE AND HUMIDITY EFFECTS ON The successful application of dope finishes on fabric depends on many things, including the method of application, temperature, humidity, correct mixture of anti-blush reducers and thinners, sanding, and preparation of the fabric. In addition to the special methods necessary in the application of dope, further precautions are required in the handling, storage, and use of dope because it is highly flammable and its fumes are harmful if breathed in excess. For the best and safest results, doping is usually done in a special dope room where many of these factors can be controlled.
Cold Effects on Dope In cold weather, dopes left in unheated rooms or outside become quite viscous (thick). Cold dopes should be kept in a warm room between 75° F. and 80° F. at least 24 hrs. before being used. Dope in large drum containers (55 gal.) will require 48 hrs.
to reach this temperature. Cold dopes will pull and rope under the brush and, if thinned sufficiently to spray or brush, will use extra thinner needlessly and will lack body when the thinner evaporates.
COMMON TROUBLES IN DOPE APPLICATION Bubbles and Blisters A heavy coat of lacquer applied over a doped surface that is not thoroughly dry will tend to form bubbles. To prevent this condition, allow the surface to dry for 10 to 12 hrs. Bubbles may be removed by washing the surface with dope thinner until smooth, allowing the surface to dry, and then sanding before refinishing. Blisters are caused by dope dripping through to the opposite tabric during application of the priming coat, as a result of excessive brushing over spars, ribs, or other parts. Dope may also seep through fittings, inspection openings, or patches, and form blisters. Extreme care should be taken to avoid blisters inasmuch as they can be removed only by cutting the fabric at the blister, and patching.
Slack Panels Slack panels are caused by loose application of the fabric, or the fabric may have been applied with proper tension but permitted to remain undoped for too long a period, thus losing its tension. Fabric slackened by remaining undoped may be tightened by the application of acetone if it is applied as soon as the slackening is noticeable.
Extremes of temperature or humidity may cause dope to dry in such a condition that the fabric becomes slack. This can be remedied by spraying on another coat of dope containing either a slow dryer, such as butyl alcohol, or a rapid dryer, such as acetone, as conditions may require.
Inconsistent Coloring Inconsistent coloring of enamels, paints, and pigmented dope, is caused by the pigments settling to the bottom of the container, thus depriving the upper portion of the vehicle of its proper percentage of pigment. If shaking the container does not distribute the pigment satisfactorily, a broad paddle or an agitator should be used to stir the mixture thoroughly.
Pinholes Pinholes in the dope film can be caused by the temperature of the dope room being too high, by not brushing the priming coat well into fabric to seal it completely, by a heavy spray coat of a mixture containing too much thinner, or by water, oil, or dirt in the air supply of the spray gun.
Blushing Blushing in dopes or lacquers is common in humid weather. This condition in cellulose nitrate and cellulose acetate dopes is caused by rapid evaporation of thinners and solvents. The evaporation lowers the temperature on the surface of the freshly doped fabric, causing condensation of moisture from the atmosphere. This moisture on the surface of the wet dope or lacquer precipitates the cellulose nitrate or cellulose acetate out of solution, thus giving the thick milky-white appearance known as blush. Of course, such a decomposed finish is of no value either in tautening or protecting the surface for any period of time. Therefore, the blush must be eliminated if the finish is to endure.
The common causes of blushing are:
(1) Temperature too low.
(2) Relative humidity too high.
(3) Drafts over freshly doped surface.
(4) Use of acetone as a thinner instead of nitrate thinner.
If causes (1) and (2) cannot be corrected, blushing may be avoided by adding butyl alcohol to the dope in sufficient quantity to correct the condition.
Dope films that have blushed may be restored by applying another coat of dope, thinned with butyl alcohol, over the blushed film. This coat will dissolve the precipitation on the previous coat. If an additional coat is not desired, the blushed film may be removed by saturating a rag with butyl alcohol and rubbing it rapidly and lightly over the blushed film. If butyl alcohol does not remove the blushing, acetone may be applied in the same manner to accomplish this purpose.
Brittleness Brittleness is caused by applying the fabric too tightly or by the aging of the doped surface. Overtight panels may be loosened by spraying a 50% solution of fast-evaporating solvent (acetone) and dope over the surface to soak into the dope layers, allowing the fabric to slacken. If the age of the doped surface causes brittleness, the only remedy is to re-cover the structure.
Peeling Peeling is caused by failure to remove moisture, oil, or grease from the fabric before the surface is coated. Fabric areas so affected should be treated with acetone before the priming coat is applied.
Runs and sags in the finish are caused either by applying the dope too heavily or by allowing the dope to run over the sides and ends of the surface.
Immediately after a surface is finished, the opposite and adjacent surfaces should be inspected for sags
TECHNIQUE OF APPLICATION Apply the first two coats of dope by brush, spread on the surface as uniformly as possible, and thoroughly work into the fabric. Be careful not to work the dope through the fabric so that an excessive film is formed on the reverse side. The first coat should produce a thorough and uniform wetting of the fabric. To do so, work the dope with the warp and the fill threads for three or four brush strokes and stroke away any excess material to avoid piling up or dripping. Apply succeeding brush or spray coats with only sufficient brushing to spread the dope smoothly and evenly.
When doping fabric over plywood or metal-cov.
ered leading edges, care should be taken to ensure that an adequate bond is obtained between the fabric and the leading edge. Care should also be taken when using predoped fabric to use a thinned dope to obtain a good bond between the fabric and the leading edge of wings.
Applying Surface Tape and Reinforcing Patches Apply surface tape and reinforcing patches with the second coat of dope. Apply surface tape over all rib lacing and all other points of the structure where tape reinforcements are required.
Installation of Drain Grommets With the second coat of dope, install drain grom- 110
mets on the underside of airfoils at the trailing edge and as close to the rib as practicable. On fuselages, install drain grommets at the center of the underside in each fuselage bay, located to ensure the best possible drainage. Special shielded grommets, sometimes called marine or suction grommets, are recommended for seaplanes to prevent the entry of spray.
Also use this type of grommet on landplanes in the part of the structure that is subject to splash from the landing gear when operating from wet and muddy fields. Plastic type grommets are doped directly to the covering. Where brass grommets are used, mount them on fabric patches and then dope them to the covering. After the doping scheme is completed, open the drainholes by cutting out the fabric with a small-bladed knife. Do not open drain grommets by punching.
Use of Fungicidal Dopes Fungicidal dope normally is used as the first coat for fabrics to prevent rotting. While it may be more advisable to purchase dope in which fungicide has already been incorporated, it is feasible to mix the fungicide with dope. Military Specification MIIL-D-7850 requires that cellulose acetate butyrate dope incorporate a fungicide for the first coat used on aircraft. The fungicide designated in this specification is zinc dimethyldithiocarbonate, which forms a suspension with the dope. This material is a fine powder, and if it is mixed with the dope, it should be made into a paste, using dope, and then diluted to the proper consistency according to the manufacturer's instructions. It is not practicable to mix the powder with a large quantity of dope.
Copper naphtonate is also used as a fungicide and forms a solution with dope. However, this material has a tendency to bleed out, especially on lightcolored fabric. It is considered satisfactory from a fungicidal standpoint.
The first coat of fungicidal dope should be applied extremely thin so that the dope can thoroughly saturate both sides of the fabric. Once the fabric is thoroughly saturated, subsequent coats may be applied at any satisfactory working consistency.
NUMBER OF COATS REQUIRED Regulations require that the total number of coats of dope should be not less than that necessary to result in a taut and well-filled finish job. A guide for finishing fabric-covered aircraft is:
(1) Two coats of clear dope, brushed on and sanded after the second coat. To prevent damaging the rib stitch lacing cords and fabric, do not sand heavily over the center portion of pinked tape over ribs and spars.
(2) One coat of clear dope, either brushed or sprayed on, and sanded.
(3) Two coats of aluminum-pigmented dope, brushed or sprayed on, and sanded after each coat.
(4) Three coats of pigmented dope (the color desired), sanded and rubbed to give a smooth glossy finish when completed.
CHAPTER 4
AIRCRAFT PAINTING AND FINISHING Metal- or wood-covered aircraft frequently are painted to protect their surfaces from deterioration and to provide a desirable finish. Many types of finishes are used on aircraft structures. Wood structures may be varnished, whereas aluminum and steel frequently are protected and preserved by applying paint. The term "paint" is used in a general sense and includes primers, enamels, lacquers, Aircraft finishes can be separated into three general classes: (1) Protective, (2) appearance, and
(3) decorative. Internal and unexposed parts are finished to protect them from deterioration. All exposed parts and surfaces are finished to provide protection and to present a pleasing appearance.
Decorative finishing includes trim striping, the painting of emblems, the application of decals, and identification numbers and letters.
FINISHING MATERIALS A wide variety of materials are used in aircraft finishing. Some of the more common materials and their uses are described in the following paragraphs.
Acetone suitable for removing grease from fabric prior to doping, for cleaning spray paint guns, and as an ingredient in paint and varnish removers. It should not be used as a thinner in dope since its rapid drying action causes the doped area to cool and collect moisture. The absorbed moisture prevents uniform drying and results in blushing.
Alconol Butyl alcohol is a slow-drying solvent that can be mixed with dope to retard drying of the dope film on humid days, thus preventing blushing. Generally, 5 to 10% of butyl alcohol is sufficient for this purpose.
Butyl alcohol and ethyl alcohol are used together as a mixture to dilute wash coat primer as necessary for spray application. The percentage of butyl alcohol used will depend on the temperature and humidity. The butyl alcohol retards the evaporation rate.
In some cases a 25% butyl/75% ethyl alcohol mixture may be satisfactory; in others, a 50/50 mixture may be required.
Denatured alcohol is used for thinning shellac to spray gun consistency, and as a constituent of paint and varnish remover.
Isopropyl alcohol is used as a diluent in the formulation of oxygen system cleaning solutions. It is also used in preparing nonionic detergent mixtures.
Benzene Benzene is used for cleaning equipment in which enamel, paint, or varnish has been used. It is also used as a constituent of paint and varnish remover.
Dopes, enamels, paints, etc., are thinned for use in spray guns, for more efficient brushing consistency, and for reducing the thickness of coats. The correct thinner must be used with a specific finishing material.
Several materials required for thinning specific paints and lacquers are also available for solvent cleaning, but they must be used with care, Most of these materials have very low flash points and, in addition, will damage existing paint finishes. Some of the more common paint thinners are briefly diecussed in the following paragraphs.
Acrylic Nitrocellulose Lacquer Thinner Acrylic nitrocellulose lacquer thinner may be effectively used to wipe small areas prior to paint touchup. It will soften the edges of the base paint film, which in turn will assure improved adhesion of the touchup coating. However, the thinner contains toluene and ketones and should never be used indiscriminately for cleaning painted surtaces.
Cellulose Nitrate Dope and Lacquer Thinner This thinner is both explosive and toxic as well as damaging to most paint finishes. It may be used 113
for hand removal of lacquer or primer overspray. It is the approved thinner for nitrocellulose lacquers and is a mixture of ketones, alcohols, and hydrocarbons.
Volatile Mineral Spirits This material is very similar to dry-cleaning solvent but evaporates somewhat faster and leaves less residue after evaporation. It can be eflectively used in wiping stripped metal surfaces just before the re-application of paint finishes. It is also used as a carrier for solvent-emulsion compounds in general cleaning.
Toluene Toluene (toluol) may be used as a paint remover in softening fluorescent finish clear topcoat sealing materials. It is also an acceptable thinner for zinc chromate primer.
Turpentine Turpentine is used as a thinner and quick-drier for varnishes, enamels, and other oil-base paints.
Turpentine is a solvent for these types of materials and can be used to remove paint spots and to clean paint brushes.
Dope Aircraft dope is essentially a colloidal solution of célulose acetate or nitrate, combined with sufficient plasticizers to produce a smooth, flexible, homogeneous film. The dope imparts to the fabric cover additional qualities of increased tensile strength, airfightness, weather-proofing, and tautness of the fabric cover. Dope must possess maximum durability, flexibility, blush resistance, and adhesion, while adding the least additional weight. Each coating of dope applied over undercoats must penetrate and = soften them, and build up a smooth, united surface without lessening the degree of fabric tautness.
The six essential constituents of dope are:
(1) Film-base compounds, which are either cellulose acetate or cellulose nitrate.
(2) Plasticizers, such as camphor oil and castor oil, used to produce a durable, flexible film.
(3) Solvents, used to dissolve the cellulosebase materials.
(4) Diluents, used for thinning the mixture.
Toxic diluents, such as benzol, are never used.
(5) Slow dryers, such as butyl alcohol, used to prevent too rapid drying, which tends to over-cool the surface, thus causing condensation of moisture and resultant blushing.
(6) Colors or pigments, which are finely ground particles of inorganic material added to clear dope to give a desired The three types of dope used for aircraft finishes are: (1) Clear, (2) semi-pigmented, and (3) pigmented. Their characteristics and uses are:
(1) There are two clear nitrate dopes. One is used to produce a gloss finish over semipigmented finishes, and as a vehicle for bronze/aluminum doped finishes. The other is a specially prepared, quick-drying material to be used only for patching.
(2) Semi-pigmented nitrate dope contains a limited quantity of pigment. It is used for finishing fabric-covered surfaces.
(3) Pigmented nitrate dope contains a greater quantity of pigment than does semi-pigmented dope, and is normally used for code marking and finishing insignia. One or two coats applied over semi-pigmented dope will produce the desired color effect.
Dope should not be applied over paint or enamel because it tends to remove such material.
Nitrocellulose Lacquer Nitrocellulose lacquers are available in both glossy and flat finishes. They are also available in either clear or pigmented form. These materials can be applied over either old type zinc chromate or the newer modified zinc chromate primer. The lacquer finish is applied in two coats; a mist coat first, with a full, wet cross coat applied within 20 to 30 min.
afterward. The lacquer finishes should be thinned as necessary, using cellulose nitrate dope and lacquer thinner. Clear lacquer may be substituted for spar varnish over doped fabric and is also used with bronze/aluminum powder to produce aluminized lacquer. Clear lacquer should never be applied over paint, enamel, or varnish because it tends to remove such material.
Acryclic Nitrocellulose Lacquer This is the most common topcoat in use today, available either in flat or glossy finish. Both types of material are required in refinishing conventional aircraft. Anti-glare areas generally require the use of flat finishes. The remaining surfaces usually are finished with glossy materials to reduce 114
heat absorption. The base materials should be thinned as necessary for spray application with acrylic nitrocellulose thinner.
Paint Drier Paint drier is added to paint when improved drying properties are desired. Excessive drier in paint will result in a brittle film, causing cracking and peeling.
Linseed Oil Linseed oil is used to reduce semi-paste colors, such as dull black stenciling paint and insignia colors, to brushing consistency. It is also used as a protective coating on the interior of metal tubing.
Zinc Chromate Primer Zinc chromate primer is applied to metallic surfaces, before the application of enamel or lacquer, as a corrosion-resistant covering and as a base for protective topcoats. Older type zinc chromate primer is distinguishable by its bright yellow color compared to the green cast of the modified primers currently used. The old type primer will adhere well to bare metal. It is still specified as an acceptable coating for internal surfaces, and it forms a part of the old type nitrocellulose system finishes. It can be applied by brush or spray and should be thinned for spraying as necessary with toluene. When this material is to be applied by brush, it should be thinned to brushing consistency with xylene to give better wet-edge retention. It dries adequately for overcoating within an hour. Zinc chromate primer is satisfactory for use under oil-base enamels or nitrocellulose lacquers. It is also an excellent dope proof paint.
Standard Wash Primer Some paint finishes in general use include a standard wash primer undercoat, also termed a metal pre-treatment coating compound. It is a twopart material consisting of resin and alcoholic phosphoric acid, which is added just prior to application. The two components should be mixed very slowly and carefully and allowed to stand at least 30 min. before use. The primer should be used within a total time of 4 hrs. Any necessary thinning is done with a 25/75 to 50/50 mixture of butyl alcohol and ethyl alcohol, respectively. The percentage of butyl alcohol used will be determined by the evaporation rate. The percentage of butyl alcohol should be kept to the minimum possible under local temperature and humidity conditions. It is particu larly important that the ratio of acid to resin in the wash primer be maintained. Any decrease in acid will result in poor coat formation. An excess of acid will cause serious brittleness.
Acrylic Cellulose Nitrate Modified Primer The lacquer primer currently applied over the wash coat base is a modified alkyd-type zinc chromate developed for its adherence to the wash primer. It does not adhere well to bare metal, but works effectively as a sandwich between the wash coat primer and the acrylic nitrocellulose topcoating. It can be thinned as necessary for spray application with cellulose nitrate thinner. In areas where the relative humidity is high, it may be more desirable to use acrylic nitrocellulose thinner. It should be topcoated within 30 to 45 min. after its application Under no condition should it dry more than an hour and a half before finish coats of acrylic lacquer are applied. If primer coats are exposed to atmospheric conditions for longer than this maximum drying period, a re-application of both the wash primer and modified primer is necessary, followed immediately by the application of the acrylic lacquer topcoat. Otherwise, complete stripping of the coatings and refinishing is required.
In general, freshly applied coatings can be removed with either acrylic lacquer thinner or methyl ethyl ketone. However, once the coat is dry, paint stripper is required for complete removal of the coating.
The finish coatings are usually applied in two coats over the modified zinc chromate primer; the first a light mist coat, and the second a wet crosscoat with 20 to 30 min. drying time allowed between the two coatings. On amphibians or seaplanes, where maximum protection is required, the finish is increased to two coats of primer and three coats of lacquer. Once the paint finish has set, paint stripper is necessary for its removal.
Enamel Enamels are special types of varnish having either an oil base or nitrocellulose base as the solvent.
Enamel finishes are generally glossy, although flat enamel finishes are available. Enameled surfaces are hard, resist scratching and the action of oils or water, and certain grades resist high temperatures.
Enamel can be applied by spraying or brushing and is suitable for either interior or exterior application.
Varnish Spar varnish is used for finishing interior or exterior wood surfaces. It produces a transparent, durable coating for use where high gloss and hardness are not the principal requirements.
Asphalt varnish is a black coating used for the protection of surfaces around lead acid batteries, and where acid or water are present.
Oil Stain Oil stain is used to stain wood for decorative purposes. It is available in light and dark shades, simulating mahogany, oak, walnut, or other wood.
Various coloring materials are used for special applications, such as insignia and signs. These colors are obtainable as pastes (powder ground in oil) to be mixed with the proper solvent.
Paint Paint is a mechanical mixture of a vehicle and a pigment. The vehicle is a liquid that cements the pigment together and strengthens it after drying.
The pigment gives solidity, color, and hardness to the paint. Among the commonly used pigments are:
iron oxide, zinc chromate, titanium oxide, iron blue lead chromate, carbon black, and chrome green.
The vehicles used for paint can be divided into two general classes: (1) Solidifying oils, and (2) volatile oils. The solidifying oils dry and become tough leathery solids upon exposure to the air.
China wood oil, tung oil, or linseed oil are the most common solidifying oils used in aircraft paint. Volatile oils, or spirits are those which evaporate when exposed. These oils are used to dilute paint to the proper consistency and to dissolve varnish resins.
The most common volatile vehicles are: Alcohol, turpentine, benzine, toluene; ethyl acetate, and butyl acetate. Paints, varnishes, and enamels are usually composed of a pigment and a mixture of both solidifying and volatile oils. Lacquer, which is noted for its rapid drying, is composed of pigments, resins, and volatile oils.
Paint Remover General-purpose paint and enamel remover is a good, nonflammable, water-rinsable paint remover.
It is used for stripping lacquer and enamel coatings from metal surfaces, and it consists of active solvents, amines, ammonia, thinners, emulsifiers, a stable chlorinated solvent, and a cresol mixture that can be applied by fluid spray or brush. The cresol additive swells the resins in the paint coatings, while the chlorinated constituents penetrate through and lift the softened resins by evaporation. This material is water-rinsable after application and can be applied several times on stubborn coatings. It should never be permitted to contact acrylic windows, plastic surfaces, or rubber products. This material should be stored indoors or in an area well protected against weather conditions. Goggle-type eyeglasses and protective clothing should be worn when using it. Paint stripping procedures, discussed later in this chapter, are the same for touchup as for a complete repainting.
Epoxy Coating Remover Strong acid solutions or alkaline tank stripping agents are the most effective materials for removal of certain well-cured epoxies at the present time, but these stripping agents may not be used on aluminimum surfaces. General-purpose paint and enamel remover can remove most epoxy finishes. Several applications or extended dwell times may be necessary for effective results.
Fluorescent Paint Remover Fluorescent paint remover, water-rinsable type, is a paint stripper designed to remove fluorescent paint finishes from exterior surfaces of aircraft.
This material is used for stripping the high-visibility coatings without affecting the permanent acrylic or cellulose nitrate coatings underneath. A permanent base coating of cellulose nitrate lacquer may be softened by this material if allowed to remain too long.
Work with paint remover should be done out of doors in shaded areas whenever practicable, or with adequate ventilation when used indoors. Rubber, plastic, and acrylic surfaces require adequate masking. Goggle type glasses, rubber gloves, aprons, and boots should be worn during any extensive application of this stripper. Hand stripping of small areas requires no special precautions.
Masking Material Masks are used to exclude areas to which dope or lacquer, etc., is not to be applied. Masks are made of thin metal, fiberboard, paper, or masking tape.
Metal and fiberboard masks are usually held in place by weights, and paper masks by masking tape.
Liquid spray shield is a solution applied to protect areas, thus serving as a liquid mask. The liquid shield and the finish deposited upon it are easily washed off with water when the design is dry.
Storage of Finishing Material Dope, paint, ename,, and other finishing material should be stored in a dry place away from direct sunlight and heat. Each container is assigned a code and color number identifying the material con- Stored paint, enamel, and other finishing material that has separated from the vehicle must be mixed to regain usefulness. If the pigment is caked, pour most of the liquid into another container and mix the caked pigment until it is free of lumps. A broad paddle or an agitator may be used for this purpose.
When the pigment is smooth and free from lumps, the liquid is added slowly and the stirring is continned to ensure complete mixing.
PAINT TOUCHUP A good intact paint finish is one of the most effective barriers available for placement between metal surfaces and corrosive media. Touching up the existing paint finish and keeping it in good condition will eliminate most general corrosion problems.
When touching up paint, confine paint coverage to the smallest area possible. Acrylic primer or lacquer may be used, but adhesion is usually poor.
Epoxy coatings, as well as the older type of zinc chromate primer, may be used for touchup on bare metal.
When a paint surface has deteriorated badly, it is best to strip and repaint the entire panel rather than attempt to touchup the area. Touchup materials should be the same as the original finish. Surfaces to be painted should be thoroughly cleaned and free from grease, oil, or moisture. Where conditions are not suitable for painting, preservatives may be used as temporary coatings until good painting conditions are restored. Paint finishes should not be too thick since thickness promotes cracking in service.
Much of the effectiveness of a paint finish and its adherence depends on the careful preparation of the surface prior to touchup and repair. It is imperative that surfaces be clean and that all soils, lubricants, or preservatives be removed.
Cleaning procedures for paint touchup are much the same as the procedures for cleaning before inspection. Many types of cleaning compounds are available. Chapter 6, Hardware, Materials, and Processes," in the Airframe and Powerplant Mechanics General Handbook, AC 65-9A, describes many of these compounds.
IDENTIFICATION OF PAINT FINISHES Existing finishes on current aircraft may be any one of several types, combinations of two or more types, or combinations of general finishes with special proprietary coatings.
Any of the finishes may be present at any given time, and repairs may have been made using materials from several different types. Some detailed information for the identification of each finish is necessary to assure adequate repair procedures. A simple test is valuable in confirming the nature of the coatings present. The following tests will aid in paint finish identification.
surface to be checked. Old nitrocellulose finishes will soften within a period of a few minutes. Acrylic and epoxy finishes will show no effects.
If not identified, next wipe down a small area of the surface in question with a rag wet with MEK (methyl ethyl ketone). MEK will pick up the pigment from an acrylic finish, but will have no effect on an epoxy coating. Wipe the surface; do not rub.
Heavy rubbing will pick up even epoxy pigment from coatings that are not thoroughly cured. Do not use MEK on nitrocellulose finishes. No test of fluorescent finishes should be necessary other than visual examination.
PAINT REMOVAL One of the most important jobs is the stripping of old paint finishes preparatory to applying a new surface cover coat. An original finish may have to be removed in any of the following cases:
(1) If a panel or other area on the aircraft has badly deteriorated paint surfaces.
(2) If repair materials are not compatible with the existing finish, thereby precluding touchup repair.
(3) If corrosion is evident or suspected under an apparently good paint coating.
The area to be stripped must be cleaned of grease, oil, dirt, or preservatives to assure maximum efficiency of the stripping compound. The selection of the type of cleaning materials to be used will depend on the nature of the matter to be removed.
Dry-cleaning solvent may be used for removing oil, grease, and soft preservative compounds. For heavyduty removal of thick or dried preservatives, other compounds of the solvent-emulsion type are available.
In general, paint stripping materials are toxic and must be used with care. The use of a generalpurpose, water-rinsable stripper is recommended for most field applications. Wherever practicable, paint removal from any large area should be done out of doors and preferably in shaded areas. If indoor removal is necessary, adequate ventilation must be assured. Synthetic rubber surfaces, including aircraft tires, fabric, and acrylics, must be thoroughly protected against possible contact with paint remover. Care must be taken when using paint remover around gas- or water-tight seam sealants, since this material will soften and destroy the integrity of the sealants.
Mask any opening that would permit stripper to get into aircraft interiors or critical cavities. Paint stripper is toxic and contains ingredients harmful to both skin and eyes, Rubber gloves, aprons of acid repellent material, and goggle type eyeglasses should be worn if any extensive paint removal is to be done. A general stripping procedure is discussed in the following paragraphs.
No prepared paint remover should be used on aircraft fabric or be allowed to come in contact with any fiberglass reinforced parts such as radomes, radio antenna, or any component such as fiberglass reinforced wheel pants or wing tips. The active agents will attack and soften the binder in these parts.
CAUTION: Any time you use a paint stripper, always wear protective goggles and rubber gloves.
If any stripper is splashed on your skin, wash it off immediately with water; and if any comes in contact with your eyes, flood them repeatedly with water and CALL A PHYSICIAN.
Brush the entire area to be stripped with a cover of stripper to a depth of 1/32 in. to 1/16 in. Any paint brush makes a satisfactory applicator, except that the bristles will be loosened by the effect of paint remover on the binder. The brush should not be used for other purposes after being exposed to paint remover.
After applying the stripping compound, it may be covered with an inexpensive polyethane drop cloth. Covering prevents rapid evaporation of the solvents and facilitates penetration of the paint film.
Allow the stripper to remain on the surface for a sufficient length of time to wrinkle and lift the paint. This may vary from 10 min. to several hours, saturated with paint remover to further loosen any finish that may still be adhering to the metal.
Re-apply the stripper as necessary in areas that remain tight or where the material has dried, and repeat the above process. Nonmetallic scrapers may be used to assist in removing persistent paint finishes.
Remove the loosened paint and residual stripper by washing and scrubbing the surface with water.
If water spray is available, use a low-to-medium pressure stream of water directly on the scrubbing broom. If steam cleaning equipment is available and the area is sufficiently large, this equipment, together with a solution of steam cleaning compound, may be used for cleaning. On small areas, any method may be used that will assure complete rin-
RESTORATION OF PAINT FINISHES The primary objective of any paint finish is the protection of exposed surfaces against deterioration. Other reasons for particular paint schemes
(1) The reduction of glare by nonspecular coatings.
(2) The use of white or light-colored, highgloss finishes to reduce heat absorption.
(3) High visibility requirements.
(4) Identification markings.
All of these are of secondary importance to the protection offered by a paint finish in good condition. A faded or stained, but well-bonded paint finish is better than a fresh touchup treatment improperly applied over dirt, corrosion products, or other contaminants.
NITROCELLULOSE LACQUER FINISHES Nitrocellulose finishes ordinarily consist of a wash primer coat and a coat of zinc chromate primer. A nitrocellulose lacquer topcoat is applied over the prime coats.
Replacement of Existing Finish When an existing nitrocellulose finish is extensively deteriorated, the entire aircraft may have to be stripped of paint and a complete new paint finish applied. When such damage is confined to one or more panels, the stripping and application of the new finish may be limited to such areas by masking to the nearest seam line.
The complete nitrocellulose lacquer finish is begun with the application of standard wash primer 118
undercoat. The wash primer should be applied in a thin coat, with the texture of the metal still visible through the coating. If absorption of water results and the coat shows evidence of blushing, successive coatings will not adhere. The area should be resprayed with butyl alcohol to re-deposit the wash primer. If blushing is still evident, it should be stripped and re-sprayed. After 20 min. drying time, adherence of the film should be checked with a thumbnail test. A moderate thumbnail scratch should not remove the prime coat.
The wash primer must be applied over a precleaned surface that has been wiped with a volatile solvent such as MEK, naphtha, or paint and lacquer thinners just before paint application. Evaporation of the solvent should be complete before the prime coat is added. Better results will be obtained if the solvent wipe-down is followed by a detergent wash.
Lacquer primer is a modified alkyd-type zinc chromate developed for its adherence to the wash primer. Lacquer primer does not adhere well to bare metal, but works effectively as a sandwich between the wash coat primer and the nitrocellulose opcoating, and can be thinned as necessary for spray application with cellulose nitrate thinner. In areas where the relative humidity is high, it may be more desirable to use acrylic nitrocellulose thinner.
For best results, lacquer primer should be topcoated within 30 to 45 min. after its application.
The old type primer will adhere well to bare metal and is still specified as an acceptable coating for internal surfaces as well as a part of the nitrocellulose finishes. Apply by brush or spray; thin for spraying with toluene. When this material is to be applied by brush, thin to brushing consistency with xylene to give better wet-edge retention. Overcoating may be applied within an hour.
Nitrocellulose lacquers are available in both glossy and nonspecular finishes. The lacquer finish is applied in two coats: a mist coat first, with a full wet crosscoat applied within 20 to 30 min. The lacquer should be thinned as necessary, using cellulose nitrate dope and lacquer thinner.
Cellulose nitrate dope and lacquer thinner (Federal Specification TT-T-266) is both explosive and toxic, as well as damaging to most paint finishes.
Dope and lacquer thinner may be used for hand removal of lacquer or primer overspray, is an approved thinner for nitrocellulose lacquers, and is a mixture of ketones, alcohols, and hydrocarbons.
The surface areas of damaged paint must be clean prior to touchup repair, and all soils, lubricants and preservatives must be removed. Cleaning procedures for paint touchup are much the same as those for paint removal.
If the old finish is not to be completely stripped, the existing surface must be prepared to receive the new cover coat after cleaning. If good adhesion is to be obtained, all loose paint should be brushed off, giving particular attention to overpaint usually found in wheel wells and wing butt areas. Curled or flaky edges must be removed and feathered to provide about 1/2 in. of overlap. A fine abrasive approved for aircraft use should be used and extreme care taken to ensure that existing surface treatments are not damaged.
After sanding, sanded areas and bare metal should be wiped with either mineral spirits, alcohol, aliphatic naphtha, or dry-cleaning solvent, Following complete evaporation of these solvents, a detergent wash using a nonionic detergent/isopropyl alcohol mixture should be applied just prior to painting. This will improve paint adhesion.
ACRYLIC NITROCELLULOSE LACQUER FINISH Acrylic nitrocellulose lacquer is one of the most common topcoats in use today, available either as nonspecular material or glossy finish. Both types of material are required in rehnishing conventional aircraft. Surfaces visible from above and other antiglare areas generally require the use of nonspecular finishes. The remaining surfaces are usually finished with glossy materials to reduce heat absorption. The base materials should be thinned as necessary for spray application with acrylic nitrocellulose thinner.
Replacement of Existing Acrylic Nitrocellulose Lacquer Finish This finish includes a wash primer cost, modified zinc chromate primer coat, and an acrylic nitrocellulose lacquer topcoat. This finish may be applied only in the sequence specified in the manufacturer's instructions and will not adhere to either the old Even when finishes are applied over old acrylic nitrocellulose coatings or the new epoxy finishes. 12W220 coatings during touchup, a softening of the old film with a compatible thinner is required.
When a finish is being rebuilt from bare metal, the steps through the application of the modified primer are the same as for nitrocellulose finishes, except that old type zine chromate primer may not be used. As with the nitrocellulose finish, the acrylic nitrocellulose topcoat should be applied within 30 to 45 min. The finish coatings are usualy 119