Check out Eastwood Internal Frame Coating. The reviews look pretty good, altho it's a bit pricey if not on sale. It will make a huge mess (paint leaking out of any seams)
I was going to try it out, but apparently the enviro nazis won't let spray cans be shipped into Alberta or something. They would sell me the nozzle tho, and it seems to fit on spray cans of tremclad rust paint. That's how I'm treating the inner welds after I finish sealing up a hollow section.
Internal Frame Coating w/Spray Nozzle
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