Airframe and Powerplant

Smooth Coats

August 1, 2006

A paint touch-up primer

Item nine under "Preventive Maintenance" in Appendix A of FAR Part 43 reads: "[Owners may] refinish decorative coatings of fuselage, wings, tail group surfaces (excluding balanced control surfaces), fairings, cowlings, landing gear, cabin, or cockpit interior when removal or disassembly of any primary structure or operating system is not required."

At first glance, it appears that this rule gives airplane owners permission to completely repaint their airplane. It's appropriate to now quote Ira Gershwin's lyric from Porgy and Bess: "It ain't necessarily so."

Bill O'Brien, the national resource manager for airworthiness at the FAA's headquarters in Washington, D.C., helped define the intent of item nine.

According to O'Brien, Appendix A is an old appendix dating back to the days of the Civil Aeronautics Administration. The term refinishing was intended to permit owners to rejuvenate butyrate dope finishes on fabric-covered airplanes. Rejuvenator solvent temporarily softens dope and restores the flexibility of the finish. This is the core of O'Brien's interpretation. He believes that if the authors of item nine wanted to permit an owner to completely repaint his aircraft, they would have spelled it out. O'Brien goes on to cite Part 43.3(d) as an overarching regulation that allows owners to legally strip and completely repaint their airplanes while working under the supervision of a certified mechanic. So a nonsupervised repaint is out, but there are many small paint repairs that owners can perform themselves.

Picking the paint, matching the color

Acrylic enamel and polyurethane-based paints such as Imron and Jet Glo will give good results. Both types are readily available and both produce glossy finishes. A word of warning is appropriate here. Polyurethane paints emit polyisocyanates, which can cause a severe respiratory allergic reaction in some people. A forced-air breathing system, although expensive, is the only solution that will allow those who are allergic to spray polyurethane paints. Acrylic enamel is very handy for touch-up and small paint-finish repairs, but the paint of choice for manufacturers and airplane paint shops is polyurethane because it combines affordability with a good-looking, durable finish.

Let's start easy by repairing a stone chip in a modern polyurethane finish. Reputable paint shops always include additional small cans of paint colors at the end of the job. Dick Guenther, the owner of Dial Eastern States Aircraft Painting in Cadiz, Ohio, said this about repairing stone chips: "Owners don't have to buy thinner or Prepsol [a cleaning product]. Just use a little rubbing alcohol to clean the area. Using a small hobby-type paintbrush or paint dabber, put a small dot of paint in the center of the bare spot — if it's at least 70 degrees Fahrenheit the paint will flow out and fill the bare spot." This simple technique is easy, takes little time, and helps maintain the paint coat integrity.

What if there are no sample cans of paint? Check in the logbooks for an entry detailing the paint job. Often paint codes are included. If codes are available, most auto paint shops will be able to mix matching paint. Obtaining the correct color is not a guarantee of a perfect paint match. The degree of oxidation and fading of the existing paint finish is directly related to time since the paint was applied and how much the paint has been exposed to the sun. It's not easy to get new paint to look the same as old paint. Fortunately, modern paints are much more resistant to fading and oxidation than earlier paints, so this is less of an issue than it used to be.

Because of the variables in paint aging, many paint professionals prefer to remove an inspection cover or fairing with the desired color for color matching. Professional painters often do their own matching — owners can get colors matched at most local auto paint supply stores. Tell the mixer you'd like enough for a pint can. Some stores also can put acrylic enamel in pressurized aerosol cans. Aerosol cans provide a low-tech method for some types of small paint repairs. (Mechanics call these "rattle cans" because a ball inside the can that promotes mixing rattles when the can is shaken.)

Two basic techniques

There are two techniques for refinishing poor or damaged paint. The easiest and quickest solution — irreverently termed a "scuff and squirt job" — is to thoroughly clean the area needing repainting, scuff the existing paint surface to remove the oxidized layer, carefully smooth (feather) out the edges of the intact paint finish until the surface feels smooth, prime the bare areas, and apply a couple of wet coats of the correct color.

The second method takes more time and costs more but there's a better chance the paint will look better and last longer. This method requires using chemical paint strippers to remove all of the existing paint that's still adhering to the metal. These stripping agents are just plain nasty — get a drop on bare skin and you'll feel it instantly; this stuff can blister skin. Wash off all of the stripper with fresh water immediately. Never apply stripping agents to fiberglass or plastic parts such as windows, fairings, or wing, elevator, and horizontal stabilizer tips. Stripping agents also are available at home supply stores, but these are often diluted compared with commercial-strength strippers. Eye protection must be worn when working with stripping agents.

Regardless of which technique you choose, spot paint repairs should be left to professionals and are beyond the scope of this article because of the high level of skill required to blend new paint into an existing finish. Because of this, field paint repairs should be planned to cover from one edge of a skin, skin lap, or joint to the other edge or joint of the skin. If a coat of paint is not carried out to the end of the skin, there will be a very noticeable ridge of paint when the masking tape is removed.

The five steps

There are five steps involved in paint repairs and the first four involve preparing the surface to receive the paint.

Cleaning is first. All the wax, oil, corrosion preventive, and Pledge must be removed. So put on a pair of latex gloves — pretend you're an airplane doctor — and wipe down the entire area with cleaning products such as Prepsol or enamel reducer (thinner) with clean, lint-free rags. If you have an air compressor and know how to safely use an air nozzle, directing compressed air into every nook and cranny will help blow out loose particles and also will dry the surfaces.

The second step is to feather the edges of the existing paint. This means sandpaper is used to taper the edges down to the layer of the primer. Feathering eliminates an abrupt and noticeable transition when the new paint is applied. Use only silicon carbide or aluminum oxide wet-or-dry sandpaper for this task — other sandpapers can introduce corrosion-causing dissimilar metals to the aluminum. Three-twenty grit is ideal for feathering paint.

Buy a rubber sanding block. These take one-quarter sheets of sandpaper — fold a sandpaper sheet in half and cut along the fold line; do it again for each piece and then stack the pieces and wrap the ends around the block. You'll end up with four pieces of paper on your block. Dip the block into water before and during sanding. This helps reduce paper clogging. When the top piece of sandpaper isn't cutting anymore, rip it off the block to expose the next piece. Carefully sand the edges until they're smooth. Here are some tips:

  • Never sand any metal off a rivet. This is easy to do, especially when sanding near round-headed rivets.
  • Sandpaper is for feathering paint.
  • Scotch-Brite pads are the tool of choice when scuffing off the oxidized top layer of existing paint.
  • Scuffing increases the grip of the primer coats.

Etching, conversion coatings, and priming

Third, more cleaning is in order. Start with a mild acid wash using a product called Alumiprep. It helps the cleaning to scrub the aluminum with a fine-textured Scotch-Brite pad while it is wet with the Alumiprep. Keep the surface wet for two to five minutes. Then rinse with clean water and let dry. Next apply a coat of Alodine 1201 (a conversion coating) with a sponge or brush, keeping it wet for two to four minutes before rinsing it with clean water. The bare aluminum will take on a gold color. Alodine increases protection against corrosion as well as increases paint-bonding. PreKote, a one-step product by Pantheon Chemical, can be used in lieu of Alumiprep/Alodine, is biodegradable, is available in quarts from airplane supply houses, and is receiving good reviews. It's more expensive than the Alumiprep/Alodine combination but it's easier to use.

The fourth step is priming. The traditional aluminum airplane primer is zinc chromate. Because there is evidence of a link between chromate pigments and lung cancer, zinc chromate is being replaced with zinc oxide. Everyone within the vicinity of any and all spray painting must always wear a respirator. This means an air-purifying respirator with replaceable charcoal filter cartridges — not a paper dust mask. These are available in the paint department of building supply stores or through some aviation supply houses. Primers are readily available in spray (rattle) cans at aircraft supply houses. Rattle-can paint is cheap, easy to obtain, and easy to use. However, airplane manufacturers and paint shops today use two-part epoxy primers. These primers are much more expensive and more bothersome but they work better. Polyurethane paints can lift some rattle-can primers but won't affect epoxy primers.

Protecting yourself

Unless paint is applied by brush or dabber, painters and helpers must always wear an air-purifying respirator and latex gloves when spraying rattle-can and epoxy paint. In addition, a pair of Tyvek painter's overalls — which can be purchased from any paint supply department — will keep the inevitable paint drips from ruining a good shirt or pair of pants.

After mixing these epoxy paints per the directions — thinners also are added at this time — set the mix aside for approximately 30 minutes before spraying. This time can be used to make sure masking tape and paper are properly applied and to lay down drop cloths to protect the floor. A final light wipe with a tack cloth will remove dust.

Two-part epoxy primers and two-part polyurethane paints must be applied by spraying. Owners who don't have access to an air compressor with an air-oil separator, air filters, and a pressure regulator have only one option when spraying two-part paints. A Preval spray kit, also available at auto paint stores, consists of a small glass jar that is screwed into the bottom of a pressurized cylinder. On top of the cylinder is a spray nozzle. Fill the jar with the mixed paint or primer and spray it on. Aircraft Spruce & Specialty, a well-known aircraft supply store, sells a similar kit called the Zolatone power spray kit. Never use newspaper for masking because the ink from the paper will transfer. Also, don't ever spend a dime on cheap masking tape; spring for the more expensive but effective blue-colored painter's tape.

Step five

Only now are you ready for step five — actually putting down the coat of color paint. Painting products work best when ambient temperatures are between 60 and 80 degrees F — stripping agents are more active, water rinses dry faster, and paints flow better.

If you haven't practiced extensively with the equipment and paint you're using, disappointment is near. A good paint finish is the product of preparation and practice.

The paint nozzle must be held about 8 inches from, and perpendicular to, the surface, and movement across the surface must be the correct speed to produce a "wet" coat. Too slow and the paint will run; too fast and the finish will be dull. Practice, practice, practice.

Professional painters lay on stripes of paint, with each layer covering approximately 50 percent of the preceding stripe. The proper technique for painting two-part epoxy or polyurethane paints on metal surfaces is to start with a single wet coat. The second wet coat should be applied with the stripes laid down perpendicular to the stripes of the first coat for full coverage. Ron Alexander of Alexander SportAir Workshops suggests that beginning painters practice until they can apply a nice, even "wet" coat of paint to a stovepipe or large-area PVC pipe without runs or dry spots. Only then will they be ready to apply a good coat of paint on all the shapes and contours of an airplane.

Two-part paints have a finite life after mixing, so mix only the amount that will be used immediately. The Poly Fiber Aircraft Coatings site expands on many of these issues.

Painting takes some skill, patience, and a commitment to preparation. Repairing paint finishes is rewarding, maintains ownership pride, and helps delay the onset of corrosion.


E-mail the author at steve.ells@aopa.org.