The sanding scratches the epoxy covers can be no finer than 80 grit. As for 2K primer, ask the supplier for some Nason 2K primer. They offer a 4:1 mix I think and it is a high build polyester or Urethane, I can't remember which. They should be able to supply you a one quart kit and the same quantity on paint and clear. That is unless they already mixed the color as a larger quantity.
As for sanding the epoxy, I usually let it dry a couple days if I'm going to sand it and then only gently so it doesn't gum the paper. i usually don't sand the epoxy, I try to get the 2K primer on it while it is still "open", that way the 2K primer sticks by chemical reaction.