Yeah, use 24 grit discs on a grinder if you want the bondo removed fast. 36 grit is good too. Then go over all the bare steel with 80 grit to get all the rust off. Then wash the bare metal with 3M Enamel Reducer# 3812. Then knock out any dents with hammer and dolly. The wash again with #3812 , then prime with epoxy primer. Do your plastic filler work over the epoxy. Sand the plastic filler smooth.Then use urethane high build primer over the plastic work.
