Depends on how many Mills ( thickness) of paint you have on the panels.
You can start with 40-80 grit , then smooth out the scratches in the metal that the coarser grits leave ,with 150-180 grit.
If you have several layers of paint on the Metal body parts ( do not use on plastic or rubber bumpers) , you can use an Air Craft stripper to remove the paint. This stuff is highly toxic , so make sure you have alot of ventalation, use thick rubber gloves as it will burn the skin, and brush it on in one direction for best results. then scrape off with a putty knife. Then you can wipe each panel down with some lacquer thinner, then sand panels with 150-180 grit.
Tape off the body seams , so you dont let the stripper run into the jambs!!!! Or remove panels and lay on saw horses .

The use PPG's DPLF epoxy Sealer. comes in several colors , black, gray, gray-green,white, red oxyide, light blue.
You can lightly sand ( when dry)the DPLF and apply your body filler over it .

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