Protection Against Deterioration By Georges Jacquemin, EAA 3618 (Reprinted in part from the magazine CANADIAN he construction of a homebuilt aircraft is a lengthy T process. At times the work drags and the builder feels that the aircraft will never get finished. Anxious to have his aircraft flying, he may be tempted to overlook an important phase of the work — protection against deterioration. Both metal and wooden structures will deteriorate rapidly if not properly protected. This protection must be planned for and accomplished during the building program if it is to be done properly. Welded tube structures must be protected against rusting, both inside and outside. The steel tubing when

purchased from the manufacturer is usually coated with a film of dried oil or some other protective agent. During welding this protection is burned off in the weld areas. Oxide then forms and eventually corrosive action spreads to the steel tubing, both inside and out. After all welding is done, a new protective film must cover the inside of the structure to protect against this corrosion. Linseed oil is generally used, and several manufacturers have produced special rusl-proofing compounds. Before welding, 1/16 in. holes should be drilled in the tubing at the joints so that all parts can be reached by the oil. The fuselage structure is then installed in a jig as shown in the accompanying drawing, and the linseed oil, or other rust-proofing compound, is introduced under pressure at the lowest point. Pressure can be obtained by gravity or by using air pressure in a tank. The oil should be heated to about 200 deg. F. to make it flow faster. The frame must be completely filled with oil and must be rotated several times to free any air bubbles and ensure that all points have been well wetted. The structure is then allowed to drain completely, rotating it several times to free oil that has been trapped. Holes which had been left open to check the flow of oil must be permanently sealed with plugs, or brass drive screws for the small holes. Corrosion may be easily detected and checked on the outside, but it may develop unnoticed on the inside

AVIATION)

and cause a sound-looking structure to fail unexpectedly. Therefore this very important phase of construction should not be overlooked or left out. The same protective procedure should be applied to other components, such as landing gear, cabane struts, wing struts, etc. The outside of all metal structures also requires careful protective preparation. Prior to applying any paint the structure must be cleaned and all traces of oxide removed. Sandblasting is the best method for this. Where sandblasting proves to be impossible or too expensive, the structure can be cleaned with emery cloth and a rotary wire brush used in an electric drill. The bare metal should not be touched by hands, otherwise corrosion will set in at these spots later. It should then be coated with a protective agent such as "Metal Prep", "Deoxidine", etc., and will then be ready for painting. The most satisfactory finish is given by spraying one coat of zinc chromate primer and a second coat of the same primer with aluminum bronze powder added to it in the ratio of 4 oz. of powder per gallon of primer. The structure can then be painted with colored lacquer if desired. When a spray gun is not available, primer and lacquer can be brushed on carefully and evenly. Small steel fittings must be protected by the same method as the main structure, with cadmium plating as an alternative. The amateur can do cadmium plating at home by the brush method. He will need a 12 volt battery, a brush, a few leads and some chemicals Aluminum fittings can be protected by one or two coats of zinc chromate^and paint. All wooden structures must be protected against the effects of humidity (mildew, etc.) which slowly attacks the glue joints. Spar varnish should be sprayed or brushed on all inside surfaces. When a part, such as a wing

leading edge, the inside of a fuselage and other enclosed boxes, is to be varnished, the varnish is applied before the last panel is glued in place. The last panel

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Drawings by Georges Jacquemin

Metallic Fuselage — Protection against corrosion 34

JANUARY

1961