Are you talking about using POR 15 on body panels? It is more useful on the frame but I suppose it would work on the underside of the body panels. Best way with the body is to cut out and weld in new. I had a bad spot on the right rear next to the tailgate. I cut out all the bad, welded in a piece of steel, then coated everything inside and out with Rust Converter then covered it with body armor. Looks good. POR 15 worked terrific on the frame and underside. Just cover every square inch of exposed skin some way because it is REALL tough to get off!