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TOPCOATS FOR FABRIC AIRPLANES – BY RON ALEXANDER from Aircraft Spruce
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TOPCOATS FOR FABRIC AIRPLANES – BY RON ALEXANDER

Choosing the proper topcoat for a fabric airplane causes more concerns and more problems than any other area. Through the years I have seen individuals complete an excellent fabric-covering job only to ruin it with the topcoat. The choice of topcoat on a fabric-covering project is very important. Once you have completed the airplane through the silver coats you are now ready to choose the type of coating you will use for the color. I emphatically state that you should use the topcoat recommended by the covering system manufacturer. Do not use automotive topcoats on fabric. They are not flexible enough for fabric airplanes and you run the risk of the topcoat cracking and peeling off within a short period of time. Most people who have covered their airplane using Ceconite fabric and dopes will continue the process and use a butyrate dope color coat. This topcoat will provide a very high gloss, durable finish. In my opinion, an airplane that has nitrate and butyrate dope throughout, if done properly, will provide an excellent finish. You can also apply one of two polyurethane color coats over butyrate. They are Randolph’s Ranthane and Poly-Fiber’s Aerothane. Both of these products are designed to be used on fabric-covered airplanes.

If you elect to use one of the polyurethane paints such as Aerothane or Ranthane, then you can spray both the fabric and the metal parts with either of these products. The metal parts should be primed using an epoxy primer prior to applying the polyurethane. This technique will certainly ensure matching color between the metal and fabric.

BUTYRATE COLOR COATS

Butyrate dope is available in a large number of colors. This is the same dope you have been spraying up to this point. You sprayed clear butyrate dope, then butyrate dope with an aluminum pigment, and now you will use butyrate dope with a color pigment. The spraying techniques are all the same. It is advisable to purchase enough butyrate color to do the entire airplane. That way you will ensure the color will be the same. It is not unusual for different batches of color butyrate to have a slightly different shade within the same color. Another way to overcome this problem is to simply open all of the cans and mix them together in a large container. You can then pour the mixture back into the original cans. This will also ensure a uniform color.

Another tip, it is very helpful to spray on two coats of white color prior to the final color you will be using. This is particularly true if you are using yellow, orange, or red as the final color. Doing this will help in reducing the transparency of these colors and they will appear more brilliant.

Steps

After you have sanded the last coat of silver and allowed it to dry, you should then mix up the butyrate color for application. You will need to thin the butyrate color using butyrate reducer on a 1 to 1 ratio. One part of reducer to one part of butyrate dope. Remember our discussion on blushing last month. You may also have to add a retarder to the mixture to prevent blushing.

After you have thoroughly mixed the dope and thinner, filter it through a paint filter into the spray gun. Clean the surface thoroughly and then use a tack cloth just prior to spraying. You should spray at least two color cross-coats onto the surface. Allow the first coat to completely dry and then wet sand it using 320 grit sandpaper before applying the second cross coat. After any sanding always clean off the sanding dust completely and let the surface dry before applying the next coat. Drying time will vary depending upon the temperature and humidity. Also, if retarder has been used the drying time will be extended.

More than likely you will want to spray on an additional 1-2 cross coats. This will depend in large part upon the color you are using, the condition of the surface, and your skill as a painter. Sand between cross coats just as before. A normal butyrate finish will consist of about 4 or more cross coats of color.

The glossiness of butyrate colors can be improved by using blush retarder. Blush retarder simply slows the drying process of the dope. The slower it dries the glossier it will appear. To achieve this gloss mix one part retarder with three parts of butyrate thinner. Then use this mixture to thin the butyrate dope one to one.

The more slowly your final coat dries the more susceptible it will be to runs, sags, and to collecting dust. Spray very carefully within a small spray booth to minimize dirt particles that are floating through the air.

Masking for Trim

Always use a high quality masking tape for trim colors. Also use butcher paper or some other type of thick paper for masking. Do not use newspaper as the print may bleed through. After you have the masking tape and paper in place, use a small squeegee and go over the tape edge carefully to press the tape onto the surface. This will help prevent any bleed through of the trim color onto the base color.

After you have everything in place, including covering the remaining parts of the airplane if necessary, spray on one coat of clear butyrate dope over the tape. This will further seal the edges of the tapes and keep the trim color dope from leaking under the tape. Spray on the necessary number of coats wet sanding between coats just as you did with the base color.

After you are satisfied with the results it is time to remove the masking tape. Be sure the trim dope has dried prior to removing the tape. As you pull the tape off keep it parallel with the surface and not perpendicular. Pull it 180 degrees away from the masked line. Let’s assume that you are pulling tape away from a trim line on the side of a fuselage. Pull the tape that is above the trim color down toward the floor. Pull the tape below the trim up toward the ceiling. The reason for doing this is to pull across the just painted trim color so that any wet paint filaments will be deposited in the same color. Be sure to pull very slowly and steadily.

POLYURETHANE COLORS

Aerothane or Ranthane polyurethane topcoats are an excellent choice over silver butyrate. Use only one of these two products over butyrate dope. Do not use an automotive urethane. One word of caution—you must use a fresh air breathing system to spray any polyurethane topcoat. A charcoal-filtered respirator is inadequate. All polyurethane paints contain chemicals that can cause sickness or even death if proper precautions are not taken. You also need to cover your skin while spraying. Use spray proof goggles to ensure you do not get it into the eyes.

Again, you must ensure the fabric is absolutely clean prior to application of this topcoat. Aerothane or Ranthane is then sprayed in multiple, light coats. Each coat must be allowed to tack up before any more wet paint is sprayed on top. Each tacky coat actually holds the subsequent coats.

Polyurethane topcoats must be catalyzed prior to using. The mix ratio is found on the directions. The best way to measure the proper amount of catalyst and base paint is to use a soup ladle. You should mix the paint and then let the chemical reaction take place for at least 20 minutes before spraying. Once the catalyst has been added you will have approximately 5-6 hours to spray the paint before it begins to thicken and harden. Therefore, only mix the amount you are going to spray.

After catalyzing the paint, you will then thin it about 10-20% using only a urethane reducer. You should start with about 10% and then if the paint begins to orange peel reduce it an additional 10%. Don’t forget to strain the paint through a paint strainer.

Just like butyrate colors, it is a good idea to paint a coat of white over the silver prior to applying the color of choice. Begin spraying the polyurethane with a very light mist coat. It should not appear wet or shiny. Before this coat dries, apply another light coat. The surface will not appear completely covered. Wait again until this coat is tacky and then spray a light, wet coat for fill and color. Let this coat dry until tacky and then shoot the final coat. To simplify, you have sprayed on one mist coat and three fill coats. Polyurethane paint will run if applied too heavily, particularly on the first coat. You also must be sure that on fill coats you are spraying into a tacky paint. That will provide the necessary adhesion.

Do not be in a hurry to apply trim colors. Allow the paint to dry at least overnight prior to any taping. After applying the trim color, remove the masking tape as soon as the trim color is dry to the touch.

CONCLUSON

In conclusion, polyurethane colors versus butyrate colors have certain advantages and disadvantages one over the other. The choice is up to you. If you want a high-gloss, durable finish that can be applied to fabric, metal, and fiberglass, then choose polyurethane. If you desire a more satin finish like those found on earlier airplanes, you may choose butyrate colors. Regardless, take time to properly prepare the surface, ensure a clean, well-lit area where you are spraying, use good spray equipment, take proper precautions against the hazards, and practice spraying before you actually begin on your airplane. All of these will help guarantee a high-quality finish on your airplane.