Ahhh, primer wars. Here’s my .02...ALCLAD does not have to be primed as it offers good corrosion protection all by itself, as long as it doesn’t get scratched. However, the ALCLAD oxide layer is thin and easily scratched, which leads many, including me, to go the primer route over everything. Some only primer the faying surfaces which the surfaces that are in contact between two components (like between a rib flange and the skin). Some don’t primer at all – Cessna didn’t until after 1994. Like others have said, a lot depends on the plane’s environment.
Here’s what some consider being the ultimate method:
1. Wash with dish soap and water – Why? To remove grease and oils.
2. Etch/Clean with alumiprep and a maroon scotchbrite pad—Why? Removes the ALCLAD layer which some primers have difficulty adhering to and removes any dirt/impurities that the soap didn’t get. ALCAD is smooth and the problem is that most primers rely on mechanical bond to adhere to the substrate. So that’s the purpose of etching and/or roughing with acid or sandpaper: to give the aluminum more tooth for the primer to mechanically bond to. However, this solution leads to another problem which is that doing this removes the ALCLAD corrosion protection, so this leads to step 3...
3. Conversion coating (ie alodine-Note very hazardous, so something similar) Why? Re-establishes an oxide layer that won’t affect how the primers mechanical bond to the aluminum
4. Prime with epoxy primer –number of coats depends on what the manufacturer says in the instructions, but regardless of how many coats, keep it thin. (Note: Epoxy has pretty much supplanted Chromates as the primer of choice)
This is not the only way to go, it’s just “a” way. For example, some just shoot a rattle can of self-etching primer (like NAPA 7220) with little prep and seem to get good results. For the best results, my advice is to follow whatever prep instructions the primer manufacturer recommends. If properly maintained, your plane will most likely outlast you in the long run regardless of whether you prime or not.
Building an RV-10 (N728TT reserved)
RV-10 N728TT (reserved), Emp/Tailcone & Wings done, working on the Fuse. Location: Dumfries, VA (suburb of DC), EAA Ch 186