I found this oil-based enamel primer which sells at Lowes for about $9 a 32 fl.oz can...
I believe this is the same paint as the Tremclad I was talking about. It's a good, cheap enamel that I spray on everything around the farm. You can spray it straight, and a single thick coat covers great on steel and doesn't run if you are any good. You can probably skip the primer as the regular one-coat enamels stick well to anything I've ever applied them to that wasn't rusty.
I've been thinking about it for airplanes for years, but you might have to thin it to avoid adding too much weight. When sprayed straight from a HVLP gun, it definitely goes on heavy.
I don't know about brushing it because I hate painting with a brush
I also hate cleaning my gun after doing small jobs, so I've considered picking up one of
these "critter" nozzle-less guns for this sort of work.