Por 15, rust oleum, tractor implement paint, what to use on rusted metal

yes as lee g said, the phosphoric acid and epoxy that contains some acid is the problem. I've used expoxy primer from TCOP Global, I believe its their " restoration" shop label ( I think). I had to use the P A because the metal was piotted and sanding didn't clean it as I wanted. I used the Phospo, le it dry completely, wiped it down with laquer thinner, primed with the TCP brand epoxy, (some refer to it as a hybrid epoxy?), went over it with urethane high build primer/ sealer, so far no problem.
did this on one car about 2 months ago, the other one 2 weeks ago.
I had a hood one a car that again, had lots of surface rust and some pittng in areas. used the Phospo, then went over it with tractor ( iol based enamel) as a temporary sealer, ( just to see what would happen).... it stuck like glue but wrinkled in spots but never lifted. the D A with 60 grit had a hard time with it! LOL
I havn't painted in last twenty years ( until this summer) as I am allergic some components. the paints, primers with low VOC , I've had no problem with. I continue to learn and ask all the shops I find as to their methods. here in southwest Missouri, there are some small oneman shops, some talented people, they paint everything from tractors, to classics, occasional ins work, ... one shop hid way back, looked in his garage and there were 340 dart, mostly street rods, not a fancy place but astonishing work...