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FILLING, PRIMING AND PAINTING
(From CP41, Page 4, July, 1984)
Paint - Primer paint
for composites. Originally RAF recommended Dupont 705 as a primer. This paint has a high
percentage of carbon black and gives excellent UV protection, but it is not the best as
far as a good base for the more expensive top coats, such as the polyurethanes. We were
recommending Dupont l00S as a replacement for 70S, because it also gave good UV protection
and much better adhesion to the top coat, but it has now been discontinued. Dupont 131S is
the recommended replacement. Any of Duponts coats, acrylic lacquer, acrylic enamel or
polyurethane (Imron) will go well with 1315.
We recommend a urethane paint over lacquer or enamel simply because the urethanes are tougher, more flexible, and stick on better. We recently painted an airplane using Ditzlers Deltron urethane. It went on well, It looks great and it is reportedly easy to repair.
Whichever top coat you decide to use, (we recommend a good brand name such as Dupont, Ditzler, Sherwin Williams, Sterling etc), we would strongly recommend that you use the particular manufacturers product from the glass structure out. In other words, you have contoured your airplane with dry micro and have gotten through the feather fill or Sterling contouring step and are ready for primer. Pick out a manufacturer such as Ditzler and use their recommended "system" from the undercoat or primer through to the top coat.
Our research has shown that this procedure will result in adequate ultra violet protection, and it will also give you, the builder, the best chance of a lasting finish that will not crack or peel. In the past, some builders have mixed manufacturers, such as Dupont primer and Sherwin Williams top coat. Normally this should work all right, but if it does not, you have no recourse to either of the manufacturers.
Final Contouring When you have
contoured your aircraft according to the finishing section, using dry micro, and are ready
for the "feather fill", here are a few suggestions.
Feather fill is a polyester product and it has been commonly recommended by RAF for over eight years. Recently we tried a few other similar products, one of them was Sterling primer/filler, which does the same job as feather fill and it is a direct substitute. To compare the two materials, feather fill is a polyester and therefore has poor adhesive qualities. It is mandatory to scratch the surface with 40 grit sand paper to allow for a mechanical bond. Feather fill works best in dry conditions such as we have here in the desert. Feather fill does not like humidity or moisture and you must not ever wet-sand feather fill. There have been a few cases of airplanes having their finish peel off in quite large pieces. The failure was at the feather fill to glass bond line, and invariably this kind of failure can be traced to moisture, high humidity conditions during application, wetsanding the feather fill etc.
Sterling primer/filler (U-1761, U-1762) on the other hand, is a urethane product. Urethanes are famous for their adhesive qualities and given a clean surface they will generally stick forever. Sanding the glass is still recommended however, as there Is nothing more disappointing than having your beautiful finish peel off! Sterling can be applied in high humidity environments, even in a pouring down rainstorm. Wet sanding is recommended. In other words, the material is essentially impervious to moisture. Sterling is more expensive than feather fill and it does seem to be slightly more prone to having pin holes after final sanding. But these can be filled with more Sterling, or 3M Spot Putty. We at RAF have used Sterling on several aircraft over the past two years and we are generally quite satisfied with it.
Sterlings biggest attribute as far as we at RAF are concerned is the fact that it cures rapidly and can usually be sanded within 45 minutes to an hour.
Recently we tried a new material (to us), Mortons Eliminator. This is a dark gray polyester type material, rather similar to feather fill. Mortons Eliminator has a few special properties that make it quite desirable. It cures quite rapidly, and the cure can be accelerated with heat. It is formulated to provide an absolute moisture or solvent barrier. Any material applied over "eliminator" will not penetrate and get under it and cause It to separate. It is designed to eliminate pin holes. It builds up well and is a good contouring medium. It sands readily once fully cured. We have not finished a complete airplane with it at this time, but we have used it on some glass parts and have been impressed by its performance.
We found that the following procedure worked best for us while using Morton's Eliminator. Sand the parts to provide a good scratch for a mechanical bond. Spray a fog coat over the entire part and allow to flash off. Spray a medium cross coat over the part and before it dries, squeegee the wet material using a soft rubber squeegee. Use firm pressure to assure that the material Is drawn Into every scratch and pin hole. Smooth the surface with the squeegee as much as possible. Allow to flash off for 15 to 20 minutes. Spray a light cross coat over the whole thing, concentrating the spray wherever it obviously needs it, such as a particularly deep scratch or dent. Allow to cure per the instructions on the can. In a 70°F environment it takes 4 hours, 90°F it takes 70 minutes. If you heat it to 150°F, it will cure in 40 minutes. Sand with 180 grit wet or dry. It is now ready for whichever primer and top coat you have decided to use.