I hope that no one minds if I jump in...I've read this thread a few times and it seems as though your problem is that the epoxy primer is not covering your filler and you have over spray issues.
Over spray happens. It can be minimized with a decrease in air pressure and gun adjustment...that being said, every time you drop air pressure from a recommended point, you loose something, atomization, flow, you get orange peel, etc...masking the vehicle is a great start.
You stated that the Epoxy primer isn't covering your filler work....I have questions, what kind of tools are you using to block your filler? What grit of paper are you using to block your filler? Are you blocking your filler? What kind of Epoxy are you using?
There is a big difference between sanding and blocking...if you sand your filler with a course grit of paper and re-prime with epoxy primer, that primer will be sucked up like a sponge. I start fairly course, and finish with 320 grit, apply another coat of primer...I expect the primer to be sucked into the filler, it's doing it's job. the fact that the primer is being sucked into the filler means that you are getting mechanical and chemical adhesion between your filler and your epoxy primer...if you finished it in course paper...re-prime and finish it in course paper....you have a cycle. So what paper are you finishing your filler/primer in before you re-prime?
If you can answer a few questions and I hope I can help.