alot of the guys on the board use POR15.... and i have never done an entire frame in it, but i did use it on a really rusty set of taillights for my stepside. i've been really happy with the seal and finish.... i didnt even have to top coat it with paint - got a nice satin gloss...
you might try a test coat on a piece of scrap, and see how the finish looks to you, cause once you put the stuff on, it aint comin off... you can buy it in really really small cans to test it out.... use a throw away brush, because you cant clean out the brush afterwards.... (i used a throw-away sponge brush).... plan on throwing out the can of POR15 after use because it will be so hardened that you cannot use the same can again... (thats why they sell it in such small cans)...
hope that helps!
jewels.