for the body guys

I think you will find that a good epoxy has such strong adhering qualities. Body filler over that. Filler can need time to shrink. Filler should always be sealer afterward shrink too. Some fillers are way better than others. Cleaning any metal with an acid can react with some epoxies. If the metal had been rusted, espcilaly pitted, I take to bare metal, use flap disc/wire cup, and treat with phosphoric acid. Neutralize if necessary. Be sure the acid etch has been neutralized to be safe.
Any thing courser than 80 grit will leave scratches 80 can too if not carefull. Like said 80 grit gives a good tooth, but epoxy will stick to cat **** even.