I read with interest your section on finishing. I thought several things were overlooked or given inadequate attention. The first, briefly mentioned, is mold release agent removal.

From : David Dale

I would start by washing the part with liquid dishwashing detergent and hot water, dry with paper towels and repeat. Alcohol may remove some greasy deposits but many mold release agents are not soluble or only slightly soluble in alcohol. Rubber cement thinner, available in art supply and office supply stores, will remove release agents without attacking foam. Test it first, but the only things I've found it will attack are rubber cement and tape adhesives, printer's ink and waxy substances. If a part is not porous, DuPont PrepSol will remove all traces of silicones and waxes left after washing with soap and water. Although I haven't tested it, I would assume it will attack modeling foams except EPP. Sanding to remove release agents does not work. It grinds the release agent into the underlying base.

Once the part is clean it needs, as you suggest, to be given some tooth. The comments I read suggest abrasives far too fine to do a good job. Most paint manufacturers discourage anything finer than 320 for final sanding. Finer grits leave the surface too slick for good adhesion. Paints grip the roughness rather than creating an intermolecular bond like epoxy does. Even epoxies respond well to a roughened surface.

I agree that the initial coat or two of paint need to be very light. They should be allowed to "tack up" (get real sticky) between coats. After the second very light coat gets tacky a single full, wet coat should be applied. It shouldn't be any fuller and wetter than required to make the paint flow, but the purpose at this point is to cause the final coat to flow and produce a glossy coat. It is not necessay to make a finish heavy for it to be good.

I once did a carefully controlled finish of a hand launched glider fuselage. The owner wanted it to look good above being light. It was a full fuselage HLG, not a pod and tailboom model. The fuselage was cleaned, weighed, sanded and primed. The primer was sanded until it was very transparent. I sprayed four coats of metallic maroon Imron thinned 10% and sprayed at the very high pressure of 80 psi. This produces lots of overspray mist but also atomizes the paint very finely. The first 2 coats were applied as described above. letting them dry intil very sticky. The third coat was applied for a glossy finish. Unfortunately, this particular color is extremely transparent and a fourth coat was required to get an even color. The total weight gain was 5.4 grams. That's one fifth of an ounce. Too much for the most extreme weight fanatics, but very light for the results achieved. More opaque colors would have needed just one tack coat and a finish coat. Conventional enamels respond well to the thinning and higher than normal spraying pressure, yielding similar results. Imron or any other 2 part polyurethane is too dangerous for the casual painter and should only be used by those properly educated in the safety procedures for such toxic products.

Your most weight concious customers won't want paint anyway. Those who do may benefit from the notes presented here. If you can use some of this information, please do so. My goal is to share my own enjoyment of model airplanes.

My modeling background is supplanted by a stint as a full scale aircraft mechanic where I worked on fabric and dope and composite aerobatic aircraft as well as a couple of restorations- a Bellanca Citabria and a Boeing Stearman.

David Dale