[BruthaMan]

Can't say for sure. When I did mine, I applied over bare metal after using Metal Prep for 30 minutes, then thoroughly drying with an air nozzle on my compressor.

The idea was, to have the cleanest surface possible where the putty the putty would stick very well, then sand it smooth and follow up with a few coats of POR-15 for permanent protection. The POR-15 prevents any moisture from getting in.

I haven't used phosphorous under POR products, so can't say for sure. I can't imagine the phosphorous would cause any problems. Once the POR-15 is coated over the putty, it's not going anywhere. POR-15 is hard as rock too.

One other thing that might help: When I used the putty to fill in rust spots/holes, I'd take two equal parts of the putty (one is a hardener) and roll them in my hands for a few minutes, kneading them every thirty seconds or so. Then I'd "pinch off" little bits at a time and work it into the areas. Don't try to lay the whole ball over the area, just pinch off little pieces of the putty and smooth it into the areas. It's a lot easier to work with that way. Another tip, keep a cup of water handy and stick your finger in it, then smooth the putty out. The putty is water proof, so the water won't hurt it at all, but it does help to smooth it down some.