Exhaustive tests conducted by the University of Mississippi have proven that airfoils must be fair within plus or minus 0.002 inch in 4 inches. Practical methods of construction cannot duplicate surfaces which are this accurate.

The only solution to the problem is to fill the slow spots. Deep fills add weight and take considerably more time so a little care during riveting will pay dividends in simplifying the smoothing. Use care to avoid low or high spots in rib to spar joints and skin splices. Use a 3/8 inch diameter wood or nylon dowel and hammer to bump up any depressed skin spots around rivets in the upper wing skin before riveting lower skin.

Below is a step by step procedure for smoothing the wings.

1. File off the heads of any 426 rivets that extend more than approximately 0.005 inch above the skin surface.

2. Wipe entire surface with lacquer thinner.

3. Etch all areas to be painted with Alumaprep or other reliable aluminum etching solution.

4. Fill any areas that require more than 0.50 inch of fill with a good epoxy resin.

5. Spray with a thin coat of zinc primate primer.

6. Spray with white lacquer primer surfacer.

7. Scrape on lacquer primer surfacer without thinning a teaspoon to a tablespoonful at a time. Use a piece of 0.032 inch stainless steel 3" x 12" with perfectly straight edges and slightly rounded corners to screed on the lacquer primer. Make three or four passes with each application. Each pass deposits an additional thin layer until all primer is distributed. Any free primer remaining after the fourth pass should be removed by continuing the last pass beyond the leading edge. A quick flip of the stainless screen will prevent the remaining lacquer from dripping off onto the floor. This residue is unfit for further screeding because it has become too thick and contains gritty particles. It can be salvaged by spraying other surfaces later.

8. Succeeding passes can be made in either a cordwise or spanwise direction as the developing surface indicates.

9. Keep applications thin to allow rapid drying.

10. Sand only when necessary to remove ridges with 180 grit wet or dry sandpaper.

11. Continue this process until no waves or roughness can be felt with experienced passes of the fingers. A plastic block 2" x 4" x 1/2" with 1/16" x 1" x 2" strips glued to each end as feet and with a dial indicator mounted in a hole in the center will do the job for those with an uncertain feel. Reversals of more than 0.002 inch during cordwise pass indicate more filling is required.

12. Finish by sanding with 400 grit wet or dry sandpaper. No further finish is required for aerodynamic smoothness and it is recommended that wings be left in this sanded finished condition as they are very easy to maintain and retouch. Bugs and dirt on the leading edge can be sanded of with 400 grit wet or dry sandpaper.

However, if a glossy finish is desired, lacquer can be applied. It will have to be sanded and buffed to regain a perfectly smooth surface. Dark colors are not recommended as they result in heating of the wing surface when exposed to bright summer sun. This is hard on the finish and thickness the boundary layer which will cause more drag.