Paint Adhesion to Anodized Aluminum?
I've never painted anodized aluminum before, I just finished a small project using some anodized 1/2" plate and was just wondering if a general purpose automotive primer would hold suitably to it. I could skim the anodizing off with a face mill if need be as it's not dimensionally critical. If ye say that it holds well I'll skip that step however.
Zinc chromate must be used as a primer before any paint is applied to aluminum. Other posts have been posted re getting rid of anodizing chemically. Peter
Anodizing is the best possible surface for paint adhesion. No special primers or anything like that normally associated with painting aluminum is required. Bare aluminum is highly reactive with air. It doesn't matter what you do to get the surface bare, chemically or mechanically, because it immediately begins to reform oxide to protect itself. An anodized surface is non-reactive, or inert, in atmospheric exposure. That's why its appearance stays substantially the same over time. Because it is inert there's no need to apply any chemical or primer whose purpose is to convert the surface from reactive to inert. Also, the method of "growth" of the anodized surface leaves it somewhat porous on a microscopic basis, so it has ideal properties for letting the paint "grip" the surface. As an example of how well anodized aluminum holds paint, all pre-painted architectural sheet aluminum is first anodized and then painted with no primer of any kind between the surface and the topcoat. You can typically take a piece of this painted sheet, bend it in a brake, and fold it flat on itself like a hemmed edge, and the paint will remain intact.
Try a piece and see for yourself. The same paint that will almost peel off bare aluminum in sheets will have to be blasted or sanded off an anodized surface.
I agree 110% on every part of your post, and anodizing is the very best way to prepare aluminum for paint... however if the anodized layer is sealed you can have problems with adhesion. I would clean the part good in a hot water, then a good degrease product used full strength, then etch in a sodium hydroxide solution (weak with warm water). Sodium hydroxide= red devil lye if you can still find it
A good clean anodized part, free of oils or waxes, is an ideal substrate for paint.
If the paint is applied soon after anodizing, as metlmuchr posted, it better be the color you want because grinding or blasting will be the only paint removal options.
Thanks to all for the info, the down side is all the slots, channels, pockets and edges I've milled are bare so I'll just be priming the whole thing. Duplicolor makes an etching primer that claims to be good for metal, (whatever that is?) aluminum and fiberglass. I'll give it a good cleaner and hot water bath of course. Again, I appreciate the info.