I JUST HAD MY BUMPERS CHROMED AND THE GUY THAT DID THEM TOLD ME TO USE POR-15 ON THE INSIDE. He had bead blasted them I asked if I need to do any pre and he said no Just put the paint right over the blasting
I just finished my entire frame with POR-15. I used Metal Prep on the surfaces I cleaned using a wire wheel. Anything I sand blasted got POR-15 over the blasted, bare metal (no need for a primer). Give it a few days to cure and it will be rock hard.
Follow the directions to the letter. If they say use metal prep, use it. They don't guarantee the material if you don't follow the instructions. If not put on right, the stuff is a mess to deal with. What ever you do, don't get any water in it while you are working with it, a drop of sweat will ruin a batch. Dry, dry and drier is best.
I use it a lot on the trailing arms, boxes, and diff's I rebuild and blast. I always use the Metal Ready and rinse it off after 30 minutes and let dry good before painting it. I used it on my frame with the body on and cleaned the frame. It chips off where jacks jack stands are. If I was using POR15 for a blasted bare frame I would use the same prcedures.
I have used PRO 15 several times on exhaust manifolds, I just sand blast, wipe clean and put on PRO 15 then start the engine and let it run for about 15 min. to bake it on. If you read the directions you will need to bake it on ( I think 450 degrees ) if not it will never dry. Read directions to be sure.
when using por 15 i always heat the metal before coating to drive out the moisture...if you coat with moisture present it will rust under the coating...i use a propane torch when ever i can .if you don't beleive me try heating what you think is a bone dry part with a torch and watch the moisture being driven off john b