Never Tried Por 15....but hear its great stuff. If you want to get rid of the rust, I suggest using Phosphoric Acid.
WARNING: if your rust is deep, AKA Cancer, then the Acid will eat away all rust and leave holes in its place.
I used this stuff, sprayed it on and left it over night. in the morning the rust was now a black color, the acid eats away the rust, then you just wash it down with water and there you go.
Note: The Phosphoric acid I used is not available in stores. its industrial grade, and can only be purchased if you have a chemical permit. I however found a sample, which was about 1/2 gal, and was told I could take it. But you can get a less potent mix at places like Lowes and Home Depot.
After that then I would Por 15.
