If the aluminum has been stripped of old paint, the surface must be roughened for the new primer to obtain a good "grip" and to eliminate all oxide. Veteran airplane owner/pilot, and the owner of Alexander Sport Air Workshops, Ron Alexander offers some tips. First, rough up the surface with fine sandpaper, scouring pads or aluminum wool. Alexander cautions, however, not to use steel wool or a steel brush.
After scouring, wash the surface with an acid etch, per the acid manufacturer's directions. Aluminum acid etch is a mild acid that dissolves away the oxide layer. Then, wash off the acid and apply a surface conversion. A conversion coating is a special chemical that inhibits oxide formation. Then, wash off the coating, and the aluminum is ready for priming.
If the aluminum has not previously been painted, Alexander says acid etching is not required. This eliminates an entire step. The only requirement is the conversion coating immediately before priming. A conversion coating forms a ceramic-like layer on the aluminum, allowing good primer adhesion. As with all painting, the surface should be free of dirt, oil and grease.
Alexander recommends a two-part epoxy primer. An epoxy-based primer prevents corrosion formation and forms a tough, hard coating that airplanes need. Airplanes operate in a very harsh environment, subjected to rain hitting the surface at 200 mph or more and wide temperature variations. Alexander thinks other types of primers have adverse chemical reactions with polyurethane-based paints. Other primers also can wrinkle because of chemical incompatibility. After the primer has cured, the top coats of paint can be applied, per the paint manufacturer's directions.