I've used seam sealer and POR15 both and you know what? That POR-15 expands and fills gaps. If your seams are tight, I would paint it in there and look at it after 2 coats. I think the POR-15 might do a better job of pushing itself into the gaps. With the seam sealer, I was pushing it around with rubber gloves to smooth it out for the factory look so you could probably use a little on top of your final coat of POR-15.
I just painted a block with 2 coats, then painted engine color on top of it. I'll find out how well it resists heat. The POR-15 gets real thick so put it on thin.