ok on bare metal. first before you do anything epoxy prime even before body filler. when you sand your body filler and you break through to bare metal, again epoxy prime and so on your sandwiching in your repairs this is the absolute best method of repair, that epoxy bites right into the metal once it has adhered it self to the metal it is hard to chip or scratch off. don't get me wrong you get a stone flying up at you with enough speed or force you will chip it but only after you dent it. anyway any time through your repair work you hit bare metal recoat the bare metal with epoxy when your done then you prime your work with a urathane pimer surfacer now you block sand and prep for paint. then you seal the car complete with epoxy primer. no w you can paint it, under coat it or use your por 15 but now the metal is protected in the best of all ways. for the wheelwells epoxyprime and undercoat. underside of floor epoxy prime cailk your seams and epoxy your caulk under coat if you like inside floor epoxy prime, caulk your seams epoxy your caulk and paint the body color now you have done everything to protect your repairs and the metal. this is a long process but well worth it for your investment. in a production shop you would epoxy seal only after the repairs are done
