people may/will disagree but i really don't care to much for POR. I have found that no matter how much you prep the area to be coated, if it is exposed to the elements moisture will eventually get under the POR and cause the rust to resurface.

I have always had success with aggressively cleaning the rusted area, if you can not have the area media blasted you can use a wire wheel, sand paper and scotch brite pads, clean the area with mineral spirits or denatured alcohol, treat the area with phosphoric acid like Ospho, and paint over that.

hope this helps