Most older primers are porous, and they absorb moisture like a sponge. For stripping, we used to use a sidewheel grinder and an 80 grit sanding disc shaped into a hexagon with a pair of tin snips. Use a very light touch, and just "wipe" the paint away down to bare metal with the tool running at full speed. As the cutting edge of the sandpaper dulls, trim about 1/4" off each edge to expose a fresh cutting edge. Don't bear down, or you'll thin the metal too much.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!