I did try POR-15 on the inside of a panel I could reach to apply the stuff but couldn't reach well enough to sand the rust away. I was disturbed to discover after following the instructions that the very expensive POR-15 simply peeled off in sheets, leaving the rust in the same exact condition it was prior. When I went to the paint store, they showed me a whole bunch of prep stuff from the same company that I needed to spend even more money on to insure the POR-15 worked.
I think it's misnamed. The stuff is so pricey that it should be called "POOR-15".
Anyway, I gave up, got in there with sandpaper the best I could, and then used one of these rust converters that turns whatever's left black and chemically alters it so that it's not rust anymore. In fact, the instructors at the body and paint class I take at night recommend that stuff.
Sometimes, what are you gonna do? You can't get into the inside of frame rails. If you could dip an entire frame in acid and then galvanize the whole thing inside and out, that would probably be the best. On frame rails, I wouldn't care if paint didn't stick.
