Module5_Aircraft Painting and Finishing
Items: 18 (including introduction)
Introduction
AMNT 270. Aircraft Painting and Finishing. Please proceed to the nest page within this
document.
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Metal Finishing
Preserves metal, smoothness reduces air resistance.
Paint removal:
First step in the coating process
Chemical strippers or dry stripping
Prepare the aircraft: masking tape, aluminum foil, polyethylene sheeting
Chemical Stripping. Dry Stripping. Proceed to the net page to learn more.
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Chemical Striping
To remove enamel:
Apply a heavy coat of stripper and cover it with polyethylene sheeting.
Remove old finish with plastic or wooden scraper.
Wash the surface.
Remove residue around rivets and screws with Scotch Brite, brass wire brush, or
aluminum wool -do not use steel wool.
Wipe entire surface with a solvent to remove wax.
To remove lacquer:
Use the same process as for enamel.
After the stripper has done its job, roll back a small portion of the polyethylene
sheeting.
Remove the finish in the same manner as used for enamel using a Plexiglas or hard
rubber scrapper.
Roll back another small portion and repeat.
Do not use chemical stripper on fiberglass parts.
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Dry Stripping
Plastic Media Blasting (PMB).
Finish blasted with tiny sharp-edged plastic beads.
Air pressure, bead size, shape, hardness determine amount of finish to be removed.
High degree of skill is required, far more control in the stripping process than with
chemical stripping.
No damage to metal, primer may be preserved, beads are reusable.
Paint residue is much easier to dispose of.
Can damage thin-skinned general aviation aircraft.
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Metal Finishing
Painting Prep.
Do not touch the surface with bare hands. Oil on the skins surface can contaminate the
surface prevent proper adhesion; cause any number of paint defects. Paint free surface
provides the opportunity to inspect for structural or corrosion damage.
Conversion Coating. Following surface preparation treat the surface with a conversion
coating. Chemical that forms an air tight film on the surface preventing any electrolyte
from reaching the metal. Phosphoric acid can be used to form a phosphate film on the
surface. Alodine treatment may also be used.
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Metal Finishing
Primers provide good bond between surface being finished and material used for
topcoats. Types of primers include: Zinc Chromate, Wash Primer, Epoxy Primer, Acrylic
Lacquer, Synthetic Enamel, and Polyurethane.
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Zinc Chromate
Zinc Chromate is an inhibitive primer.
Base of alkyd resin is somewhat porous.
Water enters, chromate ions are released, and held on the surface of the metal.
Prevents electrolytic action prevents corrosion to form.
Used on ferrous and non-ferrous metals.
Should be applied over a wash primer.
Tends to trap water, allowing filiform corrosion to form.
Not recommended under acrylic lacquers.
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Wash Primer
Cures quickly and allows topcoat to be sprayed on soon after application.
Sprayed on in a very thin coat.
Requires a minimum of 30 minutes to cure.
Topcoat should be applied within 8 hours
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Epoxy Primer
Best corrosion protection.
Dope proof finish, all metals and composites.
Maximum corrosion protection achieved by applying epoxy primer over wash primer.
Cure a minimum of 5 hours prior to applying acrylic lacquer or synthetic enamel.
Cure a minimum of 1 hour for polyurethanes.
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Acrylic Lacquer
Apply over wash primer or epoxy primer.
Rub it down with wadded-up kraft paper; enables topcoat bonding.
Low solid content requires careful attention to application.
Finish coat should be applied over a white base coat.
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Synthetic Enamel
Made of pigments suspended in resins that cure by oxidization.
Produce a glossy finish.
Chemical and abrasive resistance is not nearly so good as that of polyurethanes
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Polyurethane
Polyurethane is the most popular finish: chemical and abrasive resistance and high gloss
finish.
Two part, chemically cured finish contains twice the solids used in acrylic lacquer.
Slow flowing resins continue to flow after thinners evaporate, forming a perfectly flat
surface.
Apply wash primer, then epoxy primer, then the polyurethane finish.
Catalysts are highly reactive to moisture.
Do not leave cans open longer than is necessary.
Clean application equipment immediately after use.
Once hardened it is virtually impossible to remove.
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Fabric Finishing - Organic Fabric Finishes
Fill coats. Applied after fabric is completely prepared. Utility finish. Gives necessary
tautness and fill to the fabric. Necessary protection to the metal. Does not have a glossy
appearance.
Show type finish. Usually made up of many coats of dope.
Much sanding and rubbing of the surface between coats. Can also be provided by a
polyurethane finish.
Ultraviolet-blocking coats. Clear dope in which tiny flakes of aluminum metal are
suspended. Protect clear dope fill coats and the fabric. By blocking ultraviolet rays of the
sun.
Finish coats. Should be applied to all surfaces at the same time.
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Finishing Problems
Finishing problems -anything that prevents the finish from being both protective and
decorative
Poor adhesion. Improper topcoat materials or too much aluminum powder. Applying
nitrate dope over butyrate dope.
Blushing -most common dope finish problem. Humidity is high, a drop in temperature
occurs when solvents evaporate. Causes condensation that mixes with wet dope film
Causing cellulose to precipitate out. Result: chalky appearance, not strong or attractive.
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Finishing Problems
Pin holes - Dope is composed of solids and solvents. Solids remain on the surface of the
fabric and solvents evaporate. Too much heat or a draft hardens the surface and
solvents cannot escape. Bubbles form, force their way through the surface forming pits
or pinholes in the surface.
Orange peel - Dope should dry from the surface outward, if the surface dries first,
shrinkage of the body of dope will cause the surface to wrinkle.
Fisheyes -localized spots in the finish that do not dry, caused by surface contamination.
Dope roping -rough trail of dope left behind a brush when the dope is too heavy for
proper application.
Runs and sags -result from too much finish.
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Finish Application
Image by ASA, Inc. Figure 3-30 is an image of hand holding a nozzle six to ten inches
away form a surface. Hold the nozzle of the spray gun between 6 and 10 inches from
the surface being sprayed.
Spray -numerous thin coats are preferred to one thick coat. Adjusting spray pattern.
Sequence of painting. Spray edges and corners first, spray flat surfaces last.
Paint gun problems. Distorted spray pattern. Spitting -too little material in the reservoir.
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Image by ASA, Inc. Figure 3-31. When paiting the corners fo a surface, spray
paralle ot the corner first then blend this in with the rest of the surface by
spraying perpendicular to the corner.
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Image by ASA, Inc. Figure 3-32. Paint Spray Patterns. Image patterns include correct
spray pattern, insufficient atomizing air pressure, excessive atomizing air pressure,
material buldup on one side of nozzle. One wiing port hole is plugged up.
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