This project has been going on for about 5 years now. I thought it made sense to try to get all of my parts and panels finished and sealed prior to paint, and then do a final wetsand with say 600 grit just before I applied the basecoat. I am about at that stage.
My process; generally, I took everything to bare metal, replaced panels,etc. I shot unreduced epoxy on the bare metal and then applied filler and blocked everything. Sometimes I had to spray more epoxy because I sanded through. Before I applied anything, I cleaned with solvent based wax and grease remover (SPI). I sanded the epoxy with 220 and applied at least two coats of regular build primer. I finished this using 320 grit. Sometimes I had to skim with filler and reapply 2k. There are many thin layers of everything on this car, but nowhere is there more than about 1/8" of filler. Once in a while, I got carried away and feathered my filler with 400 dry. I would always scuff with a red Scotch brite before spraying. After the regular build primer was blocked to a point where I thought I was happy, I would shoot epoxy that had about 10 to 15% reducer in it. I would move on to the next panel.
I hope this makes sense. I tried to not write a book that no-one wanted to read.