EteckResos is a pro so take my opinion knowing he has 30 yrs more experience than myself. But, here in Florida, I found it easier to get the fender/panel to bare metal, clean with PRE, then shoot with epoxy primer. This would keep the fender from flash rusting while I hammer/dolly out the dents. Then I would rough up the dent and surrounding epoxy with aggressive grit paper and do my filler work. Once level, I would scuff the entire fender for another coat of epoxy and then on to 2k for final.
The main reason for this method is I would have a couple hours a weekend to work on a fender. If I left metal exposed it would flash rust in a few hours. You wouldn't have that problem in a humidity free environment though.
