I've used the NAPA brand seam sealer; it is relatively inexpensive, easy for me to get a hold of, and has held up well so far. It sets up pretty quickly, so you need to tool as you go. As far is it sticking to POR-15... you have 2 choices, apply the seam sealer before the paint has fully cured or just give it a good scuff before applying the seam sealer. Lots of people complain about how difficult POR-15 is to sand... and I'm sure it is if you are trying to level it, but just scuffing isn't bad. It takes a few more passes to get rid of the gloss. I don't use POR-15 anymore though, as I've found Zero Rust to be far easier to work with.