I think POR15 would work much better with no prep at all. I POR'd my floorboard and later when I pulled the frame I noticed just a small speck of POR15 had leaked through a hole. This stuff bonded though dirt and slight rust. Finish is rough as hell but it ain't going nowhere.

I used POR-15 on my frame and on my control arms. The stuff is as hard as a rock! The frame and all the parts were sandblasted and epoxy primed first. Nice rotator you have there, my body is going up on one soon.

I used POR 15 on the underside, subframe etc of our '67. I had the metal shiny clean, sanded with 80 grit as final step,then Marine Clean thoroughly, then Metal Ready scuffed with scotchbrite, thorough rinse and dry. End result is outstanding, did it a year ago. Did 2 coats of POR 15. Very tough durable finish.

I have had similar experiences w/POR-15, if the part to be coated is rough and rusty, the stuff bonds with it like crazy, but on smooth clean metal, it can be made to peel off. It really is designed for bonding with rusty surfaces, the rougher and rusty-er, the better. Maybe someone from POR-15 can weigh in with a better explanation of why it behaves the way it does...