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AiKcraft Builaing IF YOU'VE BEEN FOLLOWING

the December and January "Aircraft Building" articles about painting your airplane, you now have the spraying equipment and a paint booth and have reviewed the safety issues. Now, it's time to get to work. Before you can have fun shooting paint, you have to do a lot of preparation work (if you want a first-class paint job). Compared to actually painting, preparing the surface to accept the paint consumes between 80 and 90 percent of the time it takes to apply a paint job. Proper treatment of the component parts is essential—and consuming—and how you prepare the surface depends on the surface you intend to paint. After discussing the different preparation procedures, we'll investigate some of the paints you can choose from.

Preparing Wood Wood aircraft surfaces are usually covered with fabric, and you need to prepare them properly to protect them from the ravages of moisture, which cause the wood to rot. Usually you dry sand the part and then apply a two-part epoxy varnish. Using the two-part epoxy varnish is important. It's impervious to

the solvents used in fabric covering systems, which will "lift" nonepoxy varnishes off the wood. Twopart epoxy varnish works well, too, if you'll paint the wood directly, but make sure you start with a smooth surface. Sanding is the key to a smooth surface. You want everything to be smooth to the touch, and dry sandpaper is your weapon. If you must

Surface Preparation Painting Your Airplane, Part III RON ALEXANDER fill dents, dings, and other imperfections, use an epoxy filler, like SuperFil, or a wood filler. Do not use Hondo. It will shrink over time and make the imperfection reappear.

Before you can have fun shooting paint, you have to do a lot of preparation work (if you want a first-class paint job).

butyrate system, the aluminum pigment should be mixed in the final coats of butyrate dope. Apply the recommended number of cross coats (usually three) before shooting the color coats. (One cross coat means you move the spray gun north and south and then east and west.) Before you spray the color, make sure you have a smooth, clean surface. Wet sand the final aluminum coat with 400-grit wet or dry sandpaper. Then clean the surface using a paint cleaning solvent and, finally, go over the entire area with a tack cloth.

Preparing Fiberglass After sanding all fiberglass parts smooth, shoot them with a twopart epoxy primer. If you're building a composite airplane, you must fill and prime the surfaces before this step. Some of the fillers/primers used on fiberglass will suffice as the primer for the topcoat paint. But make sure this is true by consulting with the manufacturer. If so, you need no additional coating of epoxy primer.

Preparing Steel Preparing Fabric Most fabric covering systems lead you through the entire process up to applying the final color. The most important step is to make sure you have enough UV blocker on the fabric before applying the topcoat. All fabric covering systems recommend a UV block, which is an aluminum pigment, and usually it's a pre-mixed chemical such as PolyFiber's Poly-Spray. If you're using a

More than other materials, steel is susceptible to corrosion (like rust). Completely remove it before you prime the part. This goes for old paint or primer, too. If you paint over an existing topcoat, there's a chance it might hide rust in the making. Rather than dealing with it later, when it will take your plane out of service, take care of it while you're still in the building stage of your airplane's life. Sport Aviation

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Aircraft Building A sand or bead blaster is the best way to remove rust and old paint, but be careful not to pit or damage the metal. When blasting, using the proper pressure is essential. Once you've blasted the metal part clean, you must protect it within an hour or two or demon corrosion will be at work again. Make sure you have your spray equipment loaded with primer and ready to shoot before you start blasting. Before you prime your freshly blasted surface, clean it with the appropriate solution, such as PolyFiber's C-2200 Metal Surface Cleaner. It will remove the last vestiges of wax, oil, etc. that contaminate the surface—and your paint job. Always use an epoxy primer on steel parts. Two-part epoxy primers give the best protection against corrosion, in addition to providing the best subsurface for most topcoat paints.

C-2200 METAL SURFA

pure aluminum on the alloy's exposed surfaces. Alclad is very smooth, and paint doesn't stick to it well. To guarantee that the primer will adhere to Alclad, you must clean and slightly roughen it using a conversion coating such as Alodine. This chemical process creates a ceramic layer over the aluminum that coats the surface and provides adhesion. If your surface is used aluminum, use paint stripper to remove any previous primer or paint. Next, eliminate all corrosion using fine-grit sandpaper, Scotch-Brite pads, or aluminum wool. Never use steel wool or a steel brush. Once the used aluminum is free of all paint, primer, and corrosion, you need to acid etch it to remove any oil (such as human skin oil) and corrosion you can't see and to roughen the surface to give the primer a good bond. This simple process involves washing the alum i n u m with a chemical such as Poly Fiber's E-2310 Acid Etch diluted with water.,, ... , .,

further enhances primer adhesion. After this step rinse the surface again and let it dry completely. Once again, this process is only for old aluminum. New aluminum surfaces need only be treated with a conversion coating as outlined above. After you've properly cleaned and treated new or used a l u m i n u m , prime it. Use a two-part epoxy primer to protect the aluminum against corrosion and to create a good bonding surface for the topcoat paint. You can use a different type of primer, but remember that polyurethane topcoat paint often lifts or wrinkles primers (just like paint stripper would) that are not two-part epoxy. An adequate primer coat is essential in providing a bond between the metal and the f i n a l topcoat paint. Apply the primer coat according to the manufacturer's directions. Avoid shooting one heavy coat of primer. Two light coats give a better final appearance.

CLEANER

Choose an epoxy primer after you have thoroughly cleaned and removed any corrosion. PolyFlber's C-2200 Metal Surface Cleaner should be used before you prime your blasted surface.

Preparing Aluminum

Because paint must be able to "grip" or adhere to the surface, how you prepare aluminum depends on whether the metal is new or used. To protect the alloy, most aluminum has an Alclad layer, a thin coat of 98

FEBRUARY 2001

Lluma-Dyne E-2310 is an acid etch which eliminates any oil on the surface of aluminum. •]•.

Thoroughly rinse the surface after the acid etch. Then wash the surface with E-2300 Conversion Coating, a chemical that inhibits corrosion and

As a general review, before painting any surface you must clean it, remove any corrosion, and then prime it. Epoxy primers come in a variety of colors, and green, yellow, and white are the most popular. White is easier to cover with the final topcoats. It is your choice. Builders have used zinc chromate primer for years, but its popularity is decreasing with the advent of epoxies. Don't use zinc chromate if you plan to ap-

Preparation may take 80 to 90 percent of your painting time, but it is a necessity which will prove a blessing later.

coat and let it dry for a few minutes, u n t i l it is tacky to the touch. Then shoot a f u l l coat of enamel. One coat may be s u f f i c i e n t (but you can shoot a second full coat if you desire). Acrylic lacquer has a low solid content that makes it more difficult to apply. After thinning it with the proper chemical, apply a very light tack coat. An additional four to five cross coats of paint will then have to be applied, allowing about 30 ply a polyurethane topcoat paint. How much time must elapse between priming the surface and applying the topcoat depends on the brand of paint you use. (Read—and follow—the manufacturer's directions.) Usually, an epoxy primer should completely cure and harden before you apply the topcoat, and this could take several days—sometimes a week—depending on the temperature and humidity. That process takes several days depending upon the temperature and humidity. The primer then needs to be scuff sanded to give the topcoat the adhesion it needs.

Topcoat Paint

When selecting a topcoat, you have many brand names to choose from, but just a few different types of paint: enamel, lacquer, polyurethane, and dope. Enamel is a common choice, but like zinc chromate its popularity is waning. After thinning it to proper consistency with enamel thinner, on your first pass spray a light mist

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1'olyurethane paint is perhaps the most popular topcoat choice today. It's durable, gives a high gloss finish, and is resistant to chemicals. Polyurethane has a high solid content, and it cures slowly, which

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forms a very flat surface and creates the trademark high-gloss look.) You must mix polyurethane with a catalyst prior to use, and then reduce it to proper viscosity for spraying. Two full coats follow a very light tack coat. One problem inherent to polyurethane is the thickness of the applied film. If the paint film is applied too thickly, the finished surface may crack over time.

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painting, polyurethane's major problem is its toxicity. Breathing the polyurethane spray mist can lead to severe sickness—and even death. To avoid this problem you must use a forced-air breathing system such as the HobbyAir system. And protect your skin and eyes! If you are painting a fabric air-

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when painting polyester fabrics. Regardless of the surface you're

plane, you can use a specially designed polyurethane topcoat, butyrate dope, or Poly-Tone. Butyrate dope and Poly-Fiber's PolyTone are both easy to apply because they are viscous, w h i c h makes them less likely to run. Poly-Fiber also manufactures a polyurethane paint designed especially for fabric called Aerothane. We'll conclude our series on painting next month by taking a close look at what to do when you pull the spray gun's trigger.