Dave
I do all the block sanding with lacquer based primer surfacer till I'm satisfied with the results then I coat everything with epoxy primer for the final block sanding. I have been using bondo for a long time and it doesn't crack when done right unless you crash it. I use the fiberglas reinforced filler over seams and then a finish coat of regular filler to get a smooth finish. I don't put filler over primer or paint. It adheres best to bare metal.
AL