Thanks for the reply but not a grit or sanding question. I've already sprayed epoxy and done light filler work. I only use Feather Fill if i'm able to sand with 400 afterwards. I'm asking about whether to use Regular 2K after Feather Fill and before epoxy sealer.
I've found bits & pieces of info digging through the forum but felt best to just ask the direct question.
Max, sounds like this is you are pretty new to this. I can tell by the questions you are asking. So I'll try to clarify some things for you. First, the condition of the surface of the panel after you get done sanding determines what steps you do next. Oh and 400 is WAY too fine a grit to start sanding poly primer with.
The reasons to use primer are to adhere to bare metal and provide corrosion protection. Epoxy is primarily used for this. Then 2K Urethane and Poly Primers are used for build. (Not neccesarily in combination.) To fill and build millage (thickness) so that you can block (sand) out any small imperfections in the panel. The idea with using poly primer is to fill and build. You then block the "fill and build" done using a relatively coarse grit. This is done for two reasons, one it is faster, but more importantly using a coarser grit like 180 dry (not wet or dry paper) on a long block to start, helps level the surface far better than when using finer grits. Using something like 400 grit will result in a much more wavy surface than using a coarser grit. Especially with a build primer like poly primer.
After blocking the poly (with a guide coat) you would look at the panel and see if there are any low spots remaining, or highs, pinholes etc. The condition of the surface determines what your next step would be. Sometimes that means more poly to fill and build more or some like using 2K Urethane at that point. Or if there is enough material on your panel you simply go through the grits (180 dry, 320 dry 600 wet for example) and then seal.
So to restate, after you get done sanding the panel, the condition it's in determines what you do next. Apply more build primer (2K Urethane or Poly), apply a final couple of coats of epoxy to cover any metal sand throughs and fill sand scratches, or simply go through the grits on the existing surface, to ready the panel for sealer.
