You can fill pinholes with POR15. I've done it myself, and it you do it right, no filler needed! I've used it on exterior suraces (the entire chassis, front and rear bumpers, hitch, bedsides and cross sills) and it's holding up fine. The only real complaint I have is that the Chassis Coat Black didn't do as well under the UV as I would liked it to have.
Now, back to filling, I used POR15 as a base coat on a used bumper I installed (long story) and it had a few gouges and some rust pitting. I gave it a few extra coats of POR15, then went back while it was tacky and applied a coat of POR Patch. Sanded it smooth after it set up, then primed with self etching primer (I used their brand), a few coats of Tie Coat, sanded smooth, then a couple of coates of sandable hi-build and then color coat. Got a nice glossy bumper out of the deal, and you can't even tell where the rust pits were (or the gouge for that matter).
