The problem with aluminum is that it is difficult to fully remove the oxide surface with sandpaper and not embed some of it in the aluminum itself, where it can still interfere w/ adhesion of some coatings. Chromate conversion coatings (the "chem-film" I mentioned) are the standard, where, after sanding, machining, etc., you treat w/ the chromate solution to convert a thin film of aluminum to a very adherent phosphate surface that has excellent adhesion qualities for organic coatings (paint). The application of an epoxy or paint primer to a sanded surface is reliant upon only the mechanical "grip" of the cured coating to the metal and is vulnerable to details of microdebris like sanding residue, humidity, trace solvents, etc. Sometimes it may get a good grip, sometimes not. I'd agree that epoxy has a better track record at getting that initial grip than paint alone.
"Self etching" primers like Vinyl-Lux are a conversion coating, making a new chemical bond w/ the aluminum and presenting a "friendly" surface for further coatings. It's intended for only a single thin coat, because the coating reacts only at the interface between the solution and the metal. Careful, the stuff contains phosphoric acid and metals, so don't use bare hands and dispose of any waste properly. Look at the instructions for coatings on Interlux's site.
If the primer is flaking away with the topcoat, then my guess is the priming step was the problem. I would probably have used an aluminum etching primer wash, like Interlux's zinc chromate Vinyl-Lux before applying the System 3 primer. Getting good adhesion to aluminum can be a trick. Chromate primers require careful use for safety/environmental reasons, but work.
Thanks for your helpful insights.
Areas on my masts that rub other parts (e.g. sprit booms) have peeled. I still have some System3 Yacht Primer and System3 Topcoat Paint left over from painting our aluminum masts back in February 2007. Would be best to "get it right" before spending a lot of time "getting it wrong" again.
The System3 Yacht Primer and the 2-part System3 Topcoat Paint have peeled off together, so that means the primer lost its grip on the aluminum.
The area could be prepared (sanded, etc.) and a "self etching" primer, say Interlux's zinc chromate Vinyl-Lux primer, could be applied as you suggested.
My question is, "what next?": should the System3 Yacht Primer be applied over the Vinyl-Lux? Or, alternatively should System3 Topcoat Paint go directly on top of the Vinyl-Lux. It may or may not be feasible to get an answer to this question from the experts at System3 and Interlux.
You suggested putting System3 Yacht Primer on top of the Vinyl-Lux primer. Any particular reason for that suggestion?
An alternative would be to strip the aluminum mast of all coatings and start over with Vinyl-Lux followed by whatever Interlux recommends as a top coat paint.
Any further thoughts or suggestions?