Back again.... The product info sheet for PPG's DPLF says to apply fillers after 1 hr for one coat of epoxy primer & overnight for 2 coats( no sanding).I have done it this way for around 20 years.
I've heard that ,as strange as it sounds, bondo adhears better to epoxy primmer than bare steel(roughed-up w/heavy grit sand discs.)
If anyone has been to a PPG,Dupont...etc school lately, give use the lattest test results.
This has been a much debated subject by do it yourself body men for years.