I sandblast then wipe down all areas with a compound like jasco metal etch or any others than contain phosphoric acid (naval jelly does too I think). Phosphoric acid chemically changes the iron oxide into another compound that is relatively inert. I finish off the surface with a two part polyurethane, for underbody stuff a good one is a diy bedliner coat like herculiner, doesn't scratch or chip as easily as enamels or acrylics. Using a undercoating sprayer gives good results in application.