OK, I stand corrected. I saw samples of plain Por-15 coated and uncoated sheet steel dump[ed in the Mississippi River for a year. Por-15 was unscathed. Maybe it is the steel underneath.
I have a fender that was OE, factory, in 1975, and was painted like a 40s chevrolet, Many coats of lacquer. It has tiny bubbles in about a 1/2" grid. a tiny bubble every 1/2 inch, more or less. That has to be the underlying steel, perhaps coupled with not thoroughly degreased before priming. It was ok for the first 30-40 years. Weird.