RE-COVERING AIRCRAFT SURFACE WITH GLASS CLOTH Fiber glass fabrics are acceptable for re-covering or reinforcing an aircraft surface, provided the material meets the requirements of Military Specifications MIL-C-9084, MIL-Y-1140, and. MIL-G- 1140. The tensile strength of the glass cloth should be at least equivalent to the tensile strength of the fabric originally installed on the aircraft. The chemical finish of the glaas cloth should be chemically compatible with the dope or resin to be used.
Lither the blanket or envelope method of reinforcement should be used on treated fabrics that can be sewn. Untreated fabric that cannot be sewn may be applied in overlapping sections. The practices recommended for doped seams should be used.
Where the glass cloth is applied only to the upper surface of the wings for hail protection, it should wrap around the trailing edge at least 1 in. and extend from the trailing edge up to and around the leading edge and back approximately to the front spar. Before starting the work, make certain that the bonding agents used will be satisfactory. Blistering or poor adhesion can occur when using bond ing agents which are not chemically compatible with the present finish on the aircraft, or which have already deteriorated because of age. A simple means of determining this is to apply a small piece of the reinforcement cloth to the original cover, using the proposed finishing process. The test sample should be visually checked the next day for blistering or poor adhesion.
When butyrate dope is used to bond glass cloth, the finishing can be accomplished in the following manner:
(3) Install reinforcing tape and structural attachments (Class B) and dope on finishing tape (cotton is recommended); then brush in one coat of 50% thinner and 50% butyrate dope.
(4) Follow by conventional finishing schedules, which call for application of one or more coats of full-bodied clear butyrate dope, two spray coats of aluminum pigmented butyrate dope, light surface sand ing, and two spray coats of pigmented butyrate dope.
When resin is used to bond the glass cloth, after surface cleaning, the finishing may be done in the following manner:
(1) Rejuvenate the doped surface. After placing the glass cloth on the surface, brush in thoroughly a coat of resin. Saturate overlapped areas thoroughly and allow to cure.
(2) Brush in a second coat of resin smoothly and evenly and allow to cure. The finished surface should not be considered completed until all the holes between the weave of the cloth are filled flush with resin.
(3) After water sanding, paint the surface with one coat of primer surfacer and finish as desired.
(1) Thoroughly clean the surface and allow to dry. If the surface has been waxed or previously covered with other protective coatings, thoroughly remove at least the top finish coat. After placing the glass cloth on the surface, brush out smoothly and thoroughly with butyrate dope thinner and 10% (by volume) retarder.
(2) Apply a heavy coat of butyrate dope between all glass cloth overlaps. When dry, brush in butyrate rejuvenator and allow to set until the surface has again drawn tight.
Install drain grommets and inspection holes as provided in the original cover.
When using glass fabric to reinforce movable control surfaces, check to ascertain that no change has been made in their static and dynamic balance.
CAUSES OF FABRIC DETERIORATION Aircraft fabrics deteriorate more rapidly in areas of heavy industrialization than in areas that have cleaner air. The greatest single cause of aircraft fabric deterioration is sulfur dioxide. This toxic compound is present in variable amounts in the 104
New fabric patch may go entirely around 1/2 inch new airfoil if damaged on bottom surface.
fabric folded under- Trailing edge New reinforcing Rib lace folded tape on top of patch edge of patch after hand sewing.
- Rib lacing All edges folded and hand-sewn as 1/4 inch past rib y New fabric extends /Old doped stitch Detail of shown at detail cover Old doped cover Original baseball-stitched - Rib reinforcing tape back in place and rib lacing under patch
• Leading edge 1/2 inch folded under - FIGURE 3-18. Method of replacing cover.
atmosphere. It occurs in large concentrations in industrial areas. Sulfur dioxide combines with oxygen, sunlight, and moisture to form sulfuric acid, which, readily attacks cotton fabrics. Linen fabrics are also affected, but to a lesser degree than cotton.
Dacron fabric is far more resistant to sulfur dioxide and other chemicals than any other fabric except fiber glass fabric. Fiber glass fabric is not affected by moisture, mildew, chemicals, or most acids.
Mildew Mildew spores attack fabrics when they are damp. All natural cellulose fibers provide nourishment for mildew growth when conditions are right.
Mildew spores, also known as fungus or mold, can be controlled by using a fungus inhibitor. The inhibitor is usually mixed with dope and applied with the first coat of dope. Dope containing fungicides should not be sprayed because they contain poisons.
Re-covering should be done in dry, clean buildings. Damp, dirty buildings encourage mildew growth. The spores grow on damp rags, paper, etc., and are deposited directly on the fabric surfaces by any movement which stirs the air in the area.
Spores are always present in the atmosphere in varying degrees and are induced into the airframe enclosures by air movement. An aircraft should be flown frequently to circulate dry air into the wings and fuselage so that moisture, which supports mildew, will not accumulate.
Acid Dopes and Thinners The use of dopes or thinners whose acidity has increased beyond safe limits can cause rapid deterioration of aircraft fabrics. When dope is stored under extremes of heat or cold, chemical reactions increase the acidity beyond safe limits.
Stocks of Military dope compound are sold as surplus when periodic tests indicate that the dope has developed an acid content in excess of a safe value. Using surplus dope can lead to early fabric deterioration.
General-purpose thinners should not be used to thin aircraft dope. Such thinners are usually acidic and are not formulated for use with dope.
Insufficient Dope Film A thin dope film does not provide sufficient protection of the fabric from the elements, and early deterioration of the fabric may result. Ultraviolet light, which is invisible, combines with oxygen to form an oxidizing agent that attacks organic materials. The ultraviolet rays can be screened by adding pigments to the dope film and by adequately covering the fabric with the dope. Aluminum powder usually is added to two of the dope coats to stop any ultraviolet light from reaching the fabric. Undoped fabric or fabric covering that is not protected by coats of aluminum-pigmented dope should not be exposed to sunlight for long periods.
Adequate protection of the fabric usually is achieved if the dope film hides the weave of the fabric, leaving a smooth surface. This cannot be determined by the number of coats of dope applied, but rather by the dope film thickness. This varies with application technique, temperature, dope consistency, and equipment.
Cracks in the dope film admit moisture and light, causing localized deterioration of the fabric.
Storage Conditions It is generally assumed that a hangared aircraft is protected from fabric deterioration. However, premature deterioration can occur, especially on aircraft stored in an unheated hangar that has a dirt foor. During the day, sun shining on the roof raises the air temperature in the hangar. This warm air absorbs moisture from the ground. When the air cools, the absorbed moisture condenses and settles on the aircraft. Atmospheric pressure changes draw the damp air into the airframe enclosures. These conditions provide an ideal situation for promoting mildew growth.
When storing fabric-covered aircraft, all openings large enough for rodents to enter should be taped. Uric acid from mice can rot fabric. It can also corrode metal parts, such as ribs, spars, and
CHECKING CONDITION OF DOPED FABRIC The condition of doped fabric should be checked at intervals sufficient to determine that the strength of the fabric has not deteriorated to the point where airworthiness of the aircraft is affected.
The areas selected for test should be those known to deteriorate most rapidly. The top surfaces generally deteriorate more rapidly than the side or bottom surfaces. When contrasting colors are used on an aircraft, the fabric will deteriorate more rapidly under the darker colors. The dark colors absorb more heat than the lighter colors. The warmer inner surface of the fabric under the dark color absorbs more moisture from the air inside the wing or fuselage. When the surface cools, this moisture condenses and the fabric under the dark area becomes moist and promotes mildew growth in a localized area. When checking cloth fabric that has been reinforced by applying fiber glass, peel back the glass cloth in the areas to be tested. Test the underlying cloth in the conventional manner.
Checking fabric surfaces is made easier by using a fabric punch tester. There are several acceptable fabric punch testers on the market; one such tester incorporates a penetrating cone (figure 3-19).
Fabric punch testers are designed for use on the dope-finished-fabric surface of the aircraft and provide only a general indication of the degree of deterioration in the strength of the fabric covering.
Their advantage is that they may be used easily and quickly to test the fabric surfaces without cutting samples from the airplane's fabric. If a fabric punch tester indicates that the aircraft fabric strength is marginal, a laboratory test should be performed to determine the actual fabric strength.
Colored band indicator Fabric FIGURE 3-19. Fabric punch tester.
When using a punch tester similar to the one illustrated in figure 3-19, place the tip on the doped fabric. With the tester held perpendicular to the surface, apply pressure with a slight rotary action until the flange of the tester contacts the fabric. The condition of the fabric is indicated by a colorbanded plunger that projects from the top of the tester. The last exposed band is compared to a chart supplied by the manufacturer of the tester to determine fabric condition.
The test should be repeated at various positions on the fabric. The lowest reading obtained, other than on an isolated reparable area, should be considered representative of the fabric condition as a whole. Fabrics that test just within the acceptable range should be checked frequently thereafter to ensure continued serviceability.
The punch tester makes only a small hole (approximately 1/2-in. diameter) or a depression in the fabric that can be repaired quickly by doping.
on a 2-in. or 3-in. patch.
TESTING FABRIC COVERING Tensile Testing of Undoped Fabric Tensile testing of fabric is a practical means of 106
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.