While I await my new stud gun I decided to do a little reading regarding metal prep for filler (once I've pulled the dent out).
I'd wondered about the bonds between epoxy primer and filler and whether I should apply epoxy before the filler. Anyway I stumbled onto Serge's test in the classroom section http://www.autobodystore.com/filler_&_epoxy.shtml
welp, my questions are now answered and I know clearly what I'm going to do.
My little advice to boards folk... read the classroom stuff now and then... it's great!!