Building an aluminum airplane sounds like good clean fun until people start telling you what respirator you need to buy. First there's the aluminum dust from sanding and grinding. It gets everywhere, including up your nose. And then there's priming.
Aluminum seems like a pretty hardy metal, and it often is. It tends to form a thin layer of oxidation that protects it, and many of the RV parts come with an extra thick layer applied at the factory. Problem is, once you sand or scratch that layer off, it's now vulnerable to corrosion from salt water, acid rain, or other nasty things. So many people paint the inside of their airplanes with hazardous materials (particularly zinc chromate) that are illegal in many countries for their carcinogenic effects. I'm painting in my backyard and I don't want to nuke myself or my neighbors, and I don't feel like donning a bunnysuit every week.
Thankfully I stumbled on Andy Karmy's article, Primer Wars, which talks about a nontoxic alternative from Aircraft Finishing Systems. It's probably not as good as chromates, which chemically inhibit corrosion, but it does provide a boundary layer to keep that corrosive stuff off the metal. And it won't cause you to grow an extra tentacle.
Doug Fischer did some environmental testing seeming to show that it works well. I've also been soaking some painted test pieces in a tank of salt water in the backyard...the AFS pieces are looking good, the unprimed pieces are, well, looking pretty corroded.
A big aesthetic downside is that it is colored white, causing all dirt to show up really well. With so many greasy pneumatic tools around, things start looking sloppy pretty quickly.
Anyway, someone at the group mentioned a local source so I went and got some. ($7 a can) It's certainly convenient for small jobs. Flashes off very fast (less than 20 seconds in fair weather), but can be nicked easily for at least a few hours. We'll see how tough it really is tomorrow after it's cured. It does not contain any chromates and it's not clear it has much anti-corrosion capability other than acting as a barrier. It may not even be a moisture barrier according to one post.
Followup: it's not as tough as AFS but will resist a fingernail after curing overnight. I used a whole can on just my mid-fuselage, so this is not the most economical or environmentally friendly way to go. But on the plus side, I drizzled some water on a piece of cardboard with 988 on it, and it did repel the water and did not seem to absorb any at all. Talc is indeed an ingredient of 988, but it didn't seem to soak up any at all.
So, I'll probably keep on using 988 for small/quick jobs, but AFS for the big jobs and/or when I'm not in a hurry.