My personal process is on a car that needs extensive work. I will skim the filler where necessary, block that with 80/100 grit. one coat of epoxy, then poly to do a final shape in of the car, maybe spot in poly where needed. I always sand poly with 150 and higher. After that I will seal it off with one coat of epoxy and then 2k for a final prime if I feel it is ready.
I like using black epoxy in between coats of primer to basically use it as a guide when im blocking in the next stage. Plus its a good way to insure that all of my bare spots are sealed up and I don't miss any.
Sometimes I will do 2 rounds of poly. I don't generally pile it on. I stick to 3 coats of superbuild. Its a lot, but I have read where some guys go 5-6 coats..
I have done things so many ways, but this process seems to work best for me and I feel I get the best results, for me anyways.