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(2) Painting Materials.
(a) Water-Reducible Epoxy Primer, Chemical and Solvent Resistant, conforming to MIL-P-85582. A low volatile organic compound (VOC) of not more than 340 g/1), environmentally compliant primer used to improve topcoat adhesion and provide a corrosion-inhibiting undercoating. This two-part primer kit should be mixed per the paint manufacturer’s instructions and applied over a properly prepared surface as described in Chapter 4, paragraphs 403, 404, and 405, and Chapter 5, subparagraphs 502b and 503a.
NOTE: Local air pollution regulations may restrict the use of many primers, paint coatings, and thinners. Comply with all local air pollution regulations.
Most aircraft primers and topcoats are issued as two-part kits. Mix only the materials from the same two-part kit. The brand and batch numbers should be the same. Follow all mixing instructions provided by the paint manufacturer.
(b) Primer coating, Epoxy-polyamide, Chemical and Solvent Resistant, conforming to MIL-P-23377. A solvent-based primer used to improve topcoat adhesion and provide a corrosion-inhibiting undercoating. This two-part primer kit should be mixed per the paint manufacturer’s instructions and applied over a properly prepared surface described in Chapter 4, paragraphs 403, 404, and 405, and Chapter 5, subparagraphs 502b and 503a.
(c) Epoxy-polyamide, conforming to MIL-C-22750, and aliphatic polyurethane, conforming to MIL-C-83286B. Several topcoats are available for the final or finish coat. The epoxy-polyamide and aliphatic polyurethane are solvent-based topcoats used to provide long-term protection to avionics equipment. An available environmentally compliant topcoat is Coating, Polyurethane, High Solids (VOC of 420 g/l), conforming to MIL-PRF-85285, Type 1. This topcoat is a water-reduced paint that provides long-term protection to avionics equipment. These topcoats are supplied as two-part kits and should be mixed per the paint manufacturer’s instructions. Selection of a topcoat color is normally based on the equipment’s location. Unless otherwise directed, refer to the OEM specification. The topcoat should be applied over a primed surface within 24 hours of the primer application. Applicable markings should be applied, using a stencil, with Lacquer, Acrylic Nitrocellulose, Lusterless, conforming to MIL-L-19538C in an appropriate color.
WARNING: Topcoats, Epoxy-Polyamide, MIL-C-22750; Polyurethane, Aliphatic, MIL-C-83286B; and Coating, Polyurethane High Solids, MIL-PRF-85285, Type 1, are toxic to the eyes, skin, and respiratory tract.
Skin and eye protection required. Avoid repeated or prolonged contact. Use only in well-ventilated areas. Keep away from open flames or other sources of ignition.
WARNING: Prolonged breathing of vapors from organic solvents or materials containing organic solvents is dangerous. Refer to the MSDS for warnings and required protective gear. Polyurethane, Aliphatic Thinner, MIL-T-81772 is flammable and toxic to the eyes, skin, and respiratory tract.
Skin and eye protection is required. Avoid repeated or prolonged contact.
Par 504 Page 80 Use only in well-ventilated areas. Keep away from open flames or other sources of ignition.
(d) Thinners are volatile solutions used to thin or reduce the paint topcoat to a desired consistency. The type of thinner that may be used, and the recommended quantity, are determined by the paint manufacturer and displayed on the paint container.
(3) Paint Problems. Certain discrepancies may appear on finish coatings due to faulty application methods or contamination of the surface to which the coating was applied. Table 5-1 lists the most common defects, probable causes, and prevention methods.
d. Application of Coatings. The methods used to paint, touch-up, or apply preservatives depends on the extent of the job, material, tools, facilities, and time available. Spraying is faster than other methods and results in a smoother surface finish. However, the time and labor required to set up spray equipment may not be justified by the amount of work to be done, or if extensive masking is required. Brushes are used where the use of spray guns is impractical, unsuitable, or not allowed.
(1) Paint Spray Booth. The paint spray booth should be used to conduct paint spray operations. The paint spray booth should, at a minimum, be equipped with an exhaust fan and filtration system capable of evacuating paint fumes and entrapping spray particles.
(2) The paint spray booth should comply with all applicable air pollution control requirements.
e. Application of Primer. Primers identified in subparagraph 504c are to be applied on avionics equipment as follows:
(1) The surface to be primed should be thoroughly cleaned and pre-treated per instructions in Chapter 4, paragraphs 403, 404, and 405, and Chapter 5, subparagraph 502b, which describe the equipment and procedures for cleaning and corrosion removal. Surface preparation (conversion coating) for aluminum and magnesium surfaces is described in paragraph 503.
(2) Mask all openings and areas not to be painted with pressure-sensitive tape and Kraft paper or other suitable barrier materials.
(3) Mix primer per manufacturer’s instructions. Thin (reduce) with applicable thinner as required.
Par 504 Page 81 TABLE 5-1. CAUSE AND PREVENTION OF PAINT DEFECTS Paint Application Defects Cause Prevention Webbing • Insufficient thinning
• Wrong thinning mixture
• Ensure sufficient mixing and thinning Dry Spots or Dulling - flattening out of the gloss in selective areas
• First coat applied too thin, especially in hot weather
• Poorly cleaned surface
• Wrong thinning mixture
• Ensure proper application
• Ensure proper preparation of the surface
• Ensure sufficient mixing and thinning Orange Peel - orange skin appearance
• Spraying at high viscosities
• Air pressure too high
• Poor spray technique, such as spraying too far from surface
• Ensure proper thinning
• Reduce air pressure
• Move closer to surface Blistering - looks like a bubble or swelled area in the paint film
• Trapped solvents
• Corrosion under the paint film
• Moisture in air supply line
• Prolonged exposure to high humidity
• Ensure sufficient drying time between coats
• Ensure proper surface preparation
• Ensure air supply line is free of water
• Avoid use of overly fast drying thinners when temperature is high Blushing - appears like milky gray cloud on paint film
• Occurs on humid days during or shortly after application
• Caused by condensation of moisture on the freshly painted surface when it is cooled to a temperature below the dew point.
The cooling can be caused by:
a. Evaporation of fast drying solvents
b. Air from the spray gun
c. Movement of item to area where the ambient temperature is below the dew point
• Apply paint only when paint area is warm enough to prevent cooling to dew point
• Do not subject item to ambient temperatures that are below the dew point until a minimum of 2 hours has elapsed after painting Fish-Eyed - appears like small crater-like openings just after spraying
• Oil or silicone materials in the air lines
• Dirty surfaces
• Ensure all equipment and surfaces to be painted are free of oil or silicones
• To remove fish-eyed paint, clean area with an approved solvent.
Blot dry using clean absorbent cloth. Repeat as necessary and repaint.
Par 504 Page 82 Paint Application Defects Cause Prevention Grit • Settling of particulate debris onto freshly painted surfaces, usually within two hours of application
• Clean surface prior to application
• Filter paint before mixing
• Keep freshly painted surfaces protected from blowing or falling dust and debris Alligatoring - characterized by irregular separations and wide cracks in the paint film
• Softened undercoat
• Uncured layer
• Remove affected paint and refinish
• Allow adequate drying time between coats Bleeding - discoloration of the paint film
• Pigment absorption from underlying coat
• Insufficient topcoat thickness
• Apply additional topcoat. If problem remains, remove paint coatings and refinish area with specified paint system Chalking - dull, powdery film • Loss of gloss to oxidation of the topcoat
• Polishing or light sanding of area.
If problem persists, refinish affected area Checked - thin lines criss-crossing each other
• Softened undercoat
• Uncured layer
• Condition increases with aging
• Ensure sufficient mixing and thinning
• Allow adequate drying time between coats Cracking - irregular lines in the paint film
• Inadequate curing of paint
• Inadequate mixing
• Change in temperature during application
• Allow adequate drying time between coats
• Ensure sufficient mixing and thinning Crowfooting - lines branching in all directions and crossing each other
• Application before undercoat has dried
• Too rapid evaporation of thinner
• Coating too thick
• Allow adequate drying time between coats
• Ensure sufficient mixing and thinning
• Ensure proper application of paint Peeling - separation and lifting of paint
• Separation of topcoat from primer or primer from metal surface due to:
a. Lack of or improper chemical conversion coating
b. Contaminated (dirty) surface
c. Wet surface
• Ensure proper surface preparation
• Ensure surface is properly cleaned and dried Runs and Sags • Application of too much paint
• Paint contains too much solvent
• Properly apply paint
• Ensure sufficient mixing and thinning Scratches and Chips • Gouging with sharp tool
• Sharp blows by tools or stones
• Handle tools properly
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(4) Primer coat should be applied per specifications of the OEM by spraying or brushing in an even, uniform thickness. When specifications are not provided, use the following dry primer thickness limits: 1 coat, 0.0004 to 0.0010 inch thickness, for avionics equipment exposed to harsh environments apply 2 coats, 0.0008 to 0.0014 inch thickness.
(5) Ensure as much as possible that the primed item or material is kept in a dry, dust-free location until the primer coating is dry (cured) and hardened. Primer coats are normally tack-free in approximately 30 minutes and can be handled, if required, in approximately 1 hour.
The primer will be sufficiently dry (cured) to enable topcoat application in approximately 2 hours. The topcoat should be applied within 24 hours of primer application. Primer coats cured beyond the 24 hour time will require cleaning and a light coating of primer prior to topcoat application.
f. Application of Topcoat. The topcoat is the final or finish coat applied over the primer.
The type of topcoat paint and the selection of the color will depend upon the equipment’s intended location and the OEM specification. Epoxy-polyamide conforming to MIL-C-22750, aliphatic polyurethane conforming to MIL-C-46168D, Type 2, and aliphatic urethane conforming to MIL-C-83286B are topcoats that may be used. The topcoat should be applied on avionics equipment as follows:
(1) In preparation for application of a topcoat the surface should already be cleaned, pre-treated, masked, and primed per Chapter 4, paragraphs 403, 404, and 405. Surface preparation (conversion coating) for aluminum and magnesium surfaces, masking, and priming are described in paragraph 503 and subparagraph 504e.
(2) The topcoat should be applied by brush or spray to the component within 24 hours of primer application. Application method depends on the type of finish required, materials used, and the surface to be finished. Care should be taken to keep the area free of dust and other foreign matter when applying the topcoat.
(3) When a single coat does not adequately cover the primer and when protection against extreme exposure is necessary, two or more coats may be required.
(4) Remove all masking material as soon as possible.
505. ENCAPSULATES.
a. General. Encapsulates are materials used to cover a component or assembly in a continuous organic resin. Encapsulates provide electrical insulation resistance to corrosion, moisture, and fungus, and mechanically support the components. In avionics equipment, encapsulates are classified as follows:
(1) Potting Compounds. Potting compounds are used to seal electrical connectors, plugs, and receptacles.
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(2) Fungus Proof Coatings. Fungus proof coatings, usually varnishes, are used to encapsulate certain avionics circuit components in a thin protective film that is impervious to fungal attack. Varnishes were used in older microelectronics and electrical components.
(3) Conformal Coatings. Conformal coatings are used to encapsulate PCBs and modules.
b. Potting Compounds. Potting compounds are used for their moisture-proof and reinforcement properties. They are used on electrical connectors to protect against fatigue failures caused by vibration and lateral pressure at the point of wire contact with the pin. Potting compounds also protect electrical connectors from corrosion contamination and arcing by excluding moisture, stray particles, and aircraft liquids (hydraulic fluid, fuel, oil, etc.).
(1) Materials. The following materials should be used for sealing (potting) electrical connectors, plugs, and receptacles.
WARNING: Potting compounds are toxic to the eyes, skin, and respiratory tract. Skin and eye protection is required. Avoid repeated or prolonged contact. Use only in well-ventilated areas. Keep away from open flames or other sources of ignition.
(a) Sealing Compound, Synthetic Rubber, Accelerated Cure, conforming to MIL-PRF-8516 is a two-part polysulfide synthetic rubber compound kit. The kit consists of a base and an accelerator (curing agent). This sealing compound is used for sealing low-voltage electrical connectors, wiring, and other electrical apparatus where the temperature does not exceed 200 °F (93 °C). This sealing compound should not be used in engine bays, aircraft keel areas, or adjacent to bleed (hot) air ducts.
NOTE: A sealing compound that has reverted changes (reverts) from the cured compound back into a liquid compound.
(b) Sealing Compound, Silicone Rubber, room temperature vulcanizing (RTV), conforming to MIL-S-23586, Type II, Class 2, Grade B-1 is a reversion-resistant silicone rubber compound consisting of a base and an accelerator (curing agent) in a kit.
This sealing compound is used for sealing electrical connectors and components where the sustained equipment operating temperature exceeds 200 °F (93 °C). This sealant has a maximum temperature of 450 °F (232 °C). This sealant may be used in a closed space.
(c) Sealing Compound, Silicone Rubber, RTV, conforming to MIL-S-23586, Type II, Class 2, Grade A is a reversion-resistant silicone rubber compound consisting of a base and an accelerator (curing agent) in a kit. This sealant is used for sealing small electrical connectors that are not in a closed space and where the sustained equipment operating temperature exceeds 200 °F (93 °C). This sealant has a maximum temperature of 450 °F (232 °C). If electrical connectors are potted with this compound and allowed to cure in a closed space, or if they normally exist in a closed space after curing, this sealing compound will be susceptible to reversion.
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(d) Sealing Compound, Silicone Rubber, (RTV), conforming to MIL-A-46146, Type II is used for sealing electrical connectors. This compound is a less viscous (flows easily), single-component (one-part) compound. The different types of this compound are defined in Subparagraph 505d, Sealants.
(e) Sealing Compound, Fuel Resistant, conforming to SAE-AMS-S-8802 is used to prevent entry of corrosive elements, and where fuel and oil may be present. This polysulfide sealing compound consists of a base and an accelerator (curing agent) in a kit. This sealing compound has a maximum temperature of 275 °F (135 °C). The different types of this compound are defined in Subparagraph 505d, Sealants.
(f) Sealing Compound, Corrosion-Inhibiting, conforming to MIL-PRF-81733 is used to prevent entry of corrosive elements, and where fuel and oils may be present. This polysulfide sealing compound contains corrosion inhibitors, and consists of a base and an accelerator (curing agent) in a kit. This sealant has a maximum temperature of 250 °F (121 °C).
The different types of this compound are defined in Subparagraph 505d, Sealants.
(g) Sealing Compound conforming to MIL-M-24041 is used for sealing and reinforcing electrical connectors, wiring, and components in a seawater environment. This sealing compound is a two-component polyether polyurethane system consisting of a prepolymer and a curing agent. This sealing compound has good cold-flow properties and will adhere to metal, rubber, and polyvinyl chloride (PVC). The temperature range of this material is -80 °F to +300 °F.
(2) Reverting Potting Compounds. Some potting compounds have a past history of reverting to a liquid after a year or two. This reversion is highly dependent on the environment of the potting compound. Compounds that revert exhibit a sticky, viscous, oozing consistency that flows out of the connector back shell. In some cases, the reverted potting compounds flow around and through pins and receptacles, insulating the connection where continuity is required.
(3) Precautions. When using potting compounds, the following precautions should be followed:
(a) Thoroughly clean the area to be potted using an approved solvent.
(b) Follow manufacturer’s instructions when mixing the base compound and accelerator. Substitutions, partial mixing, or incorrect proportions of the base compound and accelerator may produce a sealant with inferior properties.
(c) Do not mix base compounds and accelerator components of different batch numbers because substantial electrical properties may result.
(d) Potting compounds may contain small quantities of flammable solvents, and/or may release by-products on curing. Observe all warnings and cautions, and use personal protective gear identified in the MSDS and by the potting compound manufacturer.
(e) Potting compounds that have exceeded their listed shelf life should not be used unless tested and certified by an approved laboratory.
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(f) Avoid contamination of the potting compound. Do not use masking tape and fiberboard molds around the connector during the potting compound cure. If potting molds are not furnished with the connector or are not available, a plastic sleeve should be constructed. The plastic sleeve will aid in forming the potting compound around the connector during the cure.
(g) Allow potting compounds to cure until firm prior to installing connector or components in equipment.
(h) Frozen pre-mixed potting compounds should be used as soon as possible after their removal from the freezer or a significant (up to 50 percent) reduction in working life can be experienced.
(i) Remove reverting or reverted potting as soon as possible.
c. Fungus-Proof Coatings. Fungus-proof coatings should not be applied indiscriminately to all electrical components. Treat only those components that have a specific need or are designated in the applicable manufacturer’s service directives. Fungus-proof coatings can, in some instances, be detrimental to the function and maintenance of equipment. For example, fungus-proof coatings deteriorate wire insulation, and their removal for maintenance action is labor-intensive. A repair activity should re-coat an entire area only when a touch-up of the previously coated component will not provide the required protection. Many hours of repair time will be required if the fungus-proof coating is not maintained.
(1) Fungus-Proof Varnish. The authorized fungus-proof coating is a varnish conforming to MIL-V-173C. These varnishes should be thinned, if necessary, with an approved solvent per the manufacturer’s instructions or with a thinner conforming to A-A-3007.
(2) Items That Should Not Receive Fungus-Proof Varnish. Varnish should not be applied to any surface where its application will interfere with the operation or performance of the equipment. Those surfaces should be protected during application of fungus-proof varnish with pressure-sensitive tape. The following items should be protected from varnish application:
(a) Components and Materials.
1. Cable, wire, braids, and jackets that are flexed during operation of the equipment.
2. Cables where treatment would reduce the insulation resistance below, or increase the loss factor above, the acceptable service limits provided by the OEM.
3. Variable capacitors (air, ceramic, or mica dielectric).
4. High wattage and wire wound resistors.
5. Ceramic insulators that are subject to an operating voltage over 600 volts and in danger of flashover.
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6. Painted, lacquered, or varnished surfaces, unless otherwise specified by the OEM.
7. Rotating parts such as dynamotors, generators, motors, etc. Electrical components that are associated with the rotating component may be treated in accordance with the procedures outlined in this AC.
8. Wave guides (mating surfaces).
9. Electron tubes.
10. Tube clamps.
11. Miniature tube shields.
12. Plug-in relays.
13. Pressure-contact grounds.
14. Coaxial test points or receptacles.
15. Windows, lenses, etc.
16. Transparent plastic parts.
17. Plastic materials such as polyethylene, polystyrene, polyamide, acrylic, silicone, epoxy (other than printed wiring boards), melamine, fiberglass, fluorocarbon, vinyl, and alkyds.
18. Materials used for their specific arc-resistance properties and classified as such.
(b) Electrical contacts, contact portions, or mating surfaces of binding posts. Also connectors, fuses, jacks, keys, plugs, and relay sockets (including tube sockets, switches, and test points).
(c) Mechanical Parts.
1. Bearing surfaces (including bearing surfaces of gaskets and sliding surfaces).
2. Gear teeth and gear trains or assemblies.
3. Pivots and pivot portions of hinges, locks, etc.
4. Screw threads and screw adjustments (those moved in the process of operation or adjustment).
5. Springs, except at base of pile-up.
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(d) Surfaces which rub together for electrical or magnetic contact, such as bearings, contact fingers, potentiometers, shafts, shields, and variable auto-transformers.
(e) Surfaces whose operational temperatures exceed 266 °F (130 °C), or whose operating temperature will cause carbonization or smoking.
(f) The exterior or visible outside portion of indicating instruments (do not open or treat inside), control boxes, or other equipment mounted in the cockpits of aircraft.
(3) Methods of Application. The varnish coating may be applied by spraying, brushing, dipping, or any combination of these processes. The dried film should have a clear, smooth finish (free from bubbles, wrinkles, filaments, or spray dust). Runs, puddles, or gathering of the film into drops should be avoided. The dry film thickness should be at least 0.002 inch unless otherwise directed in the manufacturer’s service directives.
NOTE: All surfaces to be coated should be free of dirt, grease, and other foreign matter. Components that cannot be cleaned satisfactorily or that show evidence of corrosion should be cleaned and treated per the instructions in this AC or replaced by acceptable components. Local air pollution regulations may restrict the use of fungus-proof varnish. Comply with all local air pollution regulations.
CAUTION: Avoid varnish adhesion failures. Apply only on clean, dry surfaces with a temperature of less than 100 °F (38 °C).
(a) Spraying. For larger equipment, a pressure-pot spray gun system with the tip regulated to give a wet spray is recommended. For small, compact equipment, a pencil spray tip, regulated to give a narrow, wet spray, is recommended. The varnish should be applied in a continuous wet coat over all parts to prevent the formation of fuzz or filaments. Incomplete wetting of the component surface with varnish will produce a dry spray. The dry spray texture appears as dust on the surface of the component. Dry spray is an unacceptable finish. The equipment or individual assembly should be sprayed from as many angles as necessary to ensure complete coverage with a wet coat. If more than one coat of varnish is necessary or required by the manufacturer’s service directives, allow sufficient drying time between coats. Refer to the varnish manufacturer’s time/temperature instructions.
(b) Brushing. All parts which cannot be reached by spray may be coated with a brush. A brush may also be used to cover small areas not covered during the spraying process.
On components that require extensive masking, brush application may prove more efficient than spray application.
(c) Dipping. Subassemblies or components may be coated by dipping, provided all requirements of subparagraphs 505c(2) and 505c(5) are met.
WARNING: Components that are to be enclosed in airtight cases (hermetically sealed) should be allowed to air-dry for at least an additional 24 hours after the varnish dries. Fumes (outgassing) may accumulate to dangerous levels within the case and may be ignited by an ignition source.
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(4) Drying of Coated Equipment. Equipment coated with varnish should be dried by heating to 130 °F (54 °C). Heating should be gradual to prevent shrinking, cracking, warping, or other deterioration of the parts or materials. The drying temperature should be maintained for at least 1/2 hour but no longer than 3 hours. The drying process may be done in a vented oven, vacuum oven, or with a hot air gun.
(5) Special Precautions. When varnish is to be applied on certain types of equipment, special precautions are required. The following paragraphs list equipment and parts requiring special precautions.
(a) Radio Receivers and Transmitters. The application of varnish will cause changes in some of the circuit constants. These changes may be discernible only by electrical tests and measurements. A change in alignment may be noted immediately after application of the varnish. As the varnish dries and ages, further changes in circuit constants may take place.
The greatest change ordinarily will occur within 72 hours after treatment. The set should be completely realigned at the end of that period.
(b) Coil Shields. When coil shields are removed and replaced, they can be damaged and alter the tuning adjustments. If there is significant damage, proper alignment may be impossible. Extreme care should be exercised in removing and installing coil shields.
(c) Trimmer Capacitors. Avoid spraying or brushing varnish on the plates of trimmer capacitors. To minimize damage, all trimmer capacitors should be masked completely during varnish application. If the variable capacitor fails to operate satisfactorily after treatment, make a thorough inspection for varnish deposits.
(d) Tuning Slugs. If varnish is accidentally applied to a tuning slug, the tuning slug will normally require replacement. Extreme care should be taken in removing and replacing tuning slugs.
(e) Discriminator Circuits. Careful adjustment of discriminator circuits after applying varnish is essential, especially in the case of frequency-modulated receivers.
Discriminator circuits are more susceptible to varnish-induced changes in circuit constants than any other component.
(f) Tuned Circuits. Be especially careful during masking to ensure the wires associated with the tuned circuit are not moved. Movement of such wires may change the circuit value.
(g) Relays. Deposits of varnish on the armature, pivot, or similar components will cause the relays to bind. Mask the entire relay carefully prior to general component varnish application. After the masking has been removed, brush-apply varnish as required to coils and leads. Relays with palladium-tipped contacts should be removed before application of varnish.
(h) Meters. Since meters are easily damaged by the varnish spray treatment, all meters should be checked for accuracy before treatment. Some meters may be affected by the heat of the drying process. In other instances, meter magnets may be affected by magnetic fields
Par 505 Page 90 that exist around drying equipment. Refer to the OEM applicable service instructions for varnish treatment. If guidance is not available, do not apply varnish.
d. Sealants. Sealants are another type of protective film used in avionics equipment.
Sealants are usually provided in the uncured form as a liquid or paste which solidifies (cures) after application. Sealants can be applied by hand methods, such as brush or spatula, or by air or mechanically powered filleting/injection guns. Sealants form a flexible seal in gaps and depressions, preventing moisture intrusion at mechanical joints, spot welds, and threaded closures. In addition, they prevent entry of corrosive environments into faying surfaces, fastener areas, exposed landing gear switches, and other metal-encased avionics equipment. They function principally as waterproof barriers. Therefore, when an inspection notes damaged sealant, it should be removed, the area inspected for damage, and new sealant applied. The following describes various types of sealant available.
(1) Sealing Compound, Polysulfide, Conforming to MIL-PRF-81733. A two-part, dichromate cured, polysulfide sealant with added soluble chromates which inhibit corrosion.
This sealant is intended for use at faying surfaces subject to galvanic action and temperatures up to 250 °F (121 °C). This sealant has excellent resistance to shrinkage, aircraft fuels, and lubricating oils. It also resists deterioration from saturation with ester-type hydraulic fluids. The mixed sealant may be applied by brush or spatula. The sealant is provided in four types which allow long assembly times.
(2) Sealing Compound, Polysulfide, Low Temperature Curing, Conforming to SAE-AMS-S-83318. A two-part polysulfide sealant characterized by an excellent cure rate at low temperature. This sealant is intended for quick field repairs to other sealants. Apply with brush.
(3) Sealing Compound, Polysulfide, Conforming to SAE-AMS-S-8802, Type A. A brush-applied, room-temperature cured sealing compound that is used for sealing gaps, seams, and faying surfaces. This sealant has excellent adhesion to most aircraft materials; excellent resistance to water, aircraft fuels (aviation gas and jet fuel), and petroleum-based oils; good resistance to deterioration from saturation with diphosphate ester-type hydraulic fluids; and can withstand temperatures up to 250 °F (121 °C). Four different sealants are available with application life and cure times from 1/2 to 4 hours. Type B sealants are spatula-applied compounds with the same characteristics as Type A.
(4) Adhesive Sealant, Silicone, RTV, Noncorrosive, Conforming to MIL-A-46146, Type III. This one-part adhesive sealant is used for encapsulating and sealing electrical and electronic components where the avionics equipment will operate at temperatures between 250 °F (121 °C) and 350 °F (177 °C).
(5) Adhesive Sealant, Silicone, RTV, Conforming to MIL-A-46146, Type I. This general purpose, one-part adhesive sealant is used for encapsulating and sealing electrical and electronic components. This material has good resistance to oxidation, weathering, and water.
CAUTION: A large number of RTV silicone sealants contain an acetic acid curing agent. These sealants, in contact with metal, result in rapid corrosion.