Got the same problem Dutch, working out in the yard, Used por 15 & tie coat primer on the last car, had some issues, this time I'm coating epoxy primer on the metal, as you are & then the light body work(metal repairs were done first)
I was actually looking at it as a seal for the metal, since all putty & primers are pourus, did my trunk lid & doors off the 73 that way & no problems to date on those parts(about 3 years)
Since most products are urathane these days I don't think there's as many issues as the old stuff, I just try & follow all the instructions & go at it! Rich.