I work on boats, so dont consider myself a body work expert. So I wont be surprised (and would welcome criticism) to my method. But, that said.....Here's what I've had good luck with. If the areas are small, wire brush thouroughly. Then follow up with a Osphoric acid "rust converter" (like Ospho, or Fozz). After at least several hours, to overnight, wipe down with fresh denatured alcohol, to be followed immediatley with an extremely thin coat of "zinc chromate" etching primer. The two part stuff is best, but I've had good results with the rattle can variety. Let sit for at least 20 minutes, no longer than a few hours, then apply the epoxy primer. I use U.S. Paints "545" Awlgrip primer. It is a two part, high build/solids available in grey or white. It will usually fill the pitting, unless they are unusually deep. Again, if they are small areas, I just use a "preval" sprayer. If youre doing a full panel, you really need a good spray rig. Fair and sand, prime over that with whatever product you are priming the rest of the vehicle with, then do your topcoat. Wherever possible, get behind the panel and apply a good coat of rubberized undercoating. I've kept serious rust in engine rooms of motor yachts controlled this way. Remember, where you have rust thru, welding a metal patch is the only way to go. I also never use polyester "bondo" type products for body filling. Only epoxy based stuff, like West System, with #410 Microlight Fairing additive (microballons). It is more expensive, but is more moisture resistant than polyesters, lighter, and will sand like butter, saving time. This stuff works for me in a marine environment, so it may sound like overkill on a truck, but I've not seen any rust come back in over four years since I adapted this method to my '82 dually diesel.
