I need to do some patch panels on a van roof. A Crash Blast video I saw recommended using either fiberglass filler or all-metal filler over welded panels before overcoating it with regular bondo filler. They said that would prevent rust from getting started since those fillers are both waterproof.
I never heard anyone recommend this before. All I’ve ever seen used was ordinary bondo fillers. Is regular filler okay or should this extra step be taken?