I bought a lead kit from Eastwood recently and it came with a video that covered both using lead and using plastic filler with no prejudice towards either. The older fellow giving the filler advice really knows his stuff and offers several good tips. I'm sure they sell the video seperately so for a few bucks it's a good read. At our shop the guys usually use epoxy primer but always do if there's any chance moisture can migrate from the rear side, like at molding or trim holes. The man from Eastwood went to bare metal after roughing with 24 grit and said to push really hard when applying the first coat of filler.

Evan