Primer Questions

Yes sand throughs into the filler. Epoxy can be used as a sealer so if you sanded through to it you can paint over it. I think a lot of people these days use a reduced epoxy as a sealer and it's not a bad idea, you just don't have to. I've done it both ways. I can't imagine the average body shop spending the extra on a sealer, definitley not for collision repairs. If you have adhesion problems it's because of prep in all probability. There's 2 ways for paint to stick, chemical bond, no sanding needed you top coated within the time window or mechanical bond, sanded to provide tooth.