Aircraft Building: The Art of Painting

usually an hour or two, and pull the tape off toward the new paint. When you're painting you will have problems, usu- ally a sag or run, and the cause is improper ...
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painting your airplane, we addressed the equipment you'll need (December) and the preparation

The Art of Painting Pulling the trigger on your airplane

you'll need to do to

achieve the best paint job (January and February). It is now time for the fun to begin—the actual spraying process itself. Before you can pull the trigger on your new spray gun, you need t o . . .


to the manufacturer's

recommendation. You can use a viscosity cup

(a cup with a small hole drilled in the bottom) for the thinning procedure, but I recommend that new painters not use it because using it can be confusing (which leads to errors and

wasted paint). Using the volume percentages the manufacturer recommends is Mix Topcoat Paint the best method for Assuming you're using thinning polyurethane a polyurethane paint, paint. The other half of you need to mix it the thinning procedure properly before you can is using the correct size shoot it onto the surf l u i d nozzle. In other words, if you thin the face. Polyurethane is a two-part paint t h a t mixture correctly and then apply it with the comes in separate cont a i n e r s : one for the wrong size nozzle, you'll encounter problems. paint and the other for Improper t h i n n i n g the catalyst. The pigments in the To insure a top-notch finish, the right equipment and work causes two problems— orange peel and runs. If paint settle during stor- area are essential. the mixture is too thick, age, so if you store the After mixing the two parts, let the paint is going on in splatters can, it's a good idea to invert it every 30 days to keep the pigment from the catalyst react with the paint for that are not thin enough to flow tocompacting at the bottom. If you at least 20 minutes before spraying. gether, and the painted surface has find a can with compacted pigment, You'll have a p p r o x i m a t e l y f i v e the same texture as an orange. If the you may need to loosen it with a hours before chemical cross-linking mixture is too thin, it runs and sags. To avoid applying these problems tool. Make sure you give the can a begins to thicken the mixed to your airplane, shoot your mixture ride in the paint shaker one week polyurethane paint. With this chemical reaction in on a test panel. If you get orange before you apply the paint, and it's a good idea to thoroughly stir it just mind, only mix the a m o u n t of peel or runs, then remix, reducing paint you'll need for the job. If you (thinning) the mixture to cure orprior to using it. Mixing the two parts of polyure- mix too much paint, cold slows the ange peel and making it more visthane is a relatively simple proce- cross-linking process, so you can cous (adding more paint/catalyst) to dure. As an example, Poly-Fiber's place it in a freezer (not with food!) cure runs. When preparing your airplane's polyurethane topcoat, Aerothane, overnight. Before you use it the has a 3-1 mix ratio, 3 parts paint to next day, let it warm to room tem- color coats, make sure your paint has the same batch number because 1 part catalyst. Measuring the flu- perature before spraying. ids in quarts, mix this ratio as acAfter mixing the two parts, you there can be slight color differences need to thin the mixture according between batches. One remedy for curately as possible. 94

MARCH 2001

Cockpit of the Future? different batch numbers is to mix all the cans together in a large container, stir well, and pour them back into their original containers. Other chemicals you may mix in your paint are a retarder, a solvent that slows the paint's drying time, or an accelerator, which speeds the drying time. Generally, you use a retarder when the temperature or humidity (or both) is high. You use an accelerator when the temperature is cool. You should only paint in temperatures between 60° F and 90° F, and you'll need a retarder or accelerator at either extreme. Finally, with paint that is not polyurethane, run it through a mesh paint strainer before loading it in the spray cup. If you're using polyurethane, strain the paint before you mix it with the catalyst.

Three words are the key to shooting a good paint job— practice, practice, practice! The Art of Painting

Three words are the key to shooting a good paint job—practice, practice, practice! And the second important thing is to not practice on your airplane (if you want a good paint job). To make sure you've got your equipment (spray gun) set up properly (which can be a learning experience in itself), practice spraying different fan patterns on several large pieces of cardboard. Once you're satisfied with the gun setup, keep practicing on the cardboard. After you've got the cardboard looking good, spray pieces of metal that are lying flat on a surface. Af-

ter you have mastered that, shoot some metal pieces hanging vertically. When you feel really confident, paint a piece of stovepipe or a large diameter PVC pipe. Stand it vertically and paint the entire piece. When you can do this without major mistakes, you're ready to

shoot your airplane—and always begin with a small control surface. A d j u s t i n g y o u r spray gun depends upon your equipment, and the manufacturer should provide instructions on setting it up. Use the spray nozzle recommended for the paint type you'll be using, and adjust the gun and shoot a test pattern on cardboard each time you paint.

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The normal spray pattern is fanshaped. To begin the actual application, hold the spray gun approximately 8 inches from the surface you're going to paint. Spreading your fingers as illustrated in Figure 1 will usually approximate this distance. This distance may vary somewhat depending on whether you're using an HVLP (high-volume, low-pressure) system or a pressure spray gun. The spray gun should be far enough away so the paint doesn't run or sag but close enough to lay on a wet coat. Holding the gun exactly perpendicular to the surface is imperative if you want to prevent the paint from being unSport Aviation


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Aircraft Building even. If you tilt the gun, the paint will be heavier on one side of the fan and lighter on the other. Move the spray gun parallel to the surface only as far as you can comfortably move your entire arm (Figure 2). If you swing your arm in an arc, the paint will be heavier in some areas and lighter in others. Squeeze the trigger just before you start the paint stroke and release it just before you finish it. To begin

the next pass, move up or down approximately half the fan width. This overlap achieves an even buildup because on each pass the gun u s u a l l y applies t h e p a i n t more thickly in the middle than on each end of the fan. Remember our definition of a cross-coat, one pass north and south followed by a pass east and west. Proper lighting is critical when painting, and arranging the lights at a 45-degree

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angle to the surface you're painting gives the best results. This lets you look into the glare, which is the best way to see that you're applying the correct amount of paint and covering all the areas. If you can't see the glare, move until you find it, just like professional painters do. Paint the edges of the structure first because they often do not receive an adequate amount of paint. To avoid runs and sags, paint surfaces on the horizontal plane whenever possible. Spraying in and around corners often presents a problem, and practice is the only way to solve it. If you're in an inside or outside corner, spray it first and then blend the paint in with subsequent strokes.

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White First Generally you should apply your white coat of paint (if you're using white) before applying other colors. This gives you better coverage with less paint, and it brings out the brilliance of the other colors. White primer serves the same purpose. Do not try to cover darker hues, especially red, with a lighter color. Red should always be the ! last coat sprayed. If you're applying more than one color, before shooting the next color let the base coat dry for the time recommended by the manufacturer, u s u a l l y at least 12 hours. Some

Poly-Fiber's polyurethane topcoat, Aerothane, has a 3:1 mix ratio, 3 parts paint to 1 part catalyst. Mix this ratio as accurately as possible.

polyurethane manufacturers recommend a week to give the paint time to cure, and then they suggest lightly sanding the area to be painted so the second color will have good tooth adhesion. When applying trim you'll need to mask the areas where you don't want the trim color to go. Use the hest grade of tape available. I would recommend 3M's fine line tape, which is designed for trim use. Do not use newspaper to cover large areas—only good Kraft paper. After you apply the tape, use a small plastic squeegee to press down the edges of the tape just prior to spraying. Remove the tape when the paint feels dry to the touch, usually an hour or two, and pull the tape off toward the new paint. When you're painting you will have problems, usually a sag or run, and the cause is improper technique, such as holding the gun too close to the surface or overthinning the paint. It happens to everybody (even the pros), so don't worry about it. When it happens, stop— and let the paint dry. Then sand out the run or sag and shoot the area again. '••;.; Orange peel is another common problem caused by under-thinned paint, using the wrong solvent, or too high an air pressure in a high-pressure spray system. If the paint blisters, you didn't prime the surface properly or moisture was present on the surface. If you have a coarse finish, you didn't clean the surface properly before painting. The solution for these situations is the same. Let the paint dry, sand it off, remedy the problem's cause, and paint the area again. The final step in painting your airplane is one that few people like—cleanup. But if you want to protect the spray gun you invested in, you must clean it thoroughly. This means taking the gun apart and cleaning it with a solvent, either a reducer or MEK. Be sure to wear gloves to protect your hands from the solvent. After cleaning the gun, some painters leave the spray nozzle in the solvent until the next job. And they always dispose of their waste materials properly. £M) Sport Aviation


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