Back in the early 80's when I was working on the frame restoration of my 46 Ply (still haven't finished the body) you could still get Zinc-Chromate paint. That's what I used, and it's what I'd use again if it is still available. (Someone made a remark about you being in aviation, so maybe you can get it, even if the average person can't.)
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I did. I'm in aviation, and I can still get zinc-chromate primer, and some zinc-phosphate as well. It's pricey stuff, but it's bulletproof. I don't know if Americans have access to a product called Endura(Canadian made), but it's also a great option. Fairly reasonably priced, and this stuff sticks. Like any paint, the key is the prepwork. I suggest MEK on bare metal before applying the primer/sealer. Then use a degreaser like FinalWipe or suitable alternatives after primer prior to topcoat. You can also get clear for endura as well if you're into shiny stuff.