400 grit on metallics is way too coarse, you will get sand scratches showing through.
On a metallic color 600 is good, I know painters using 800 grit on some colors.
If you went past the 4 day mark on the epoxy you need to sand or scuff it so you get paint adhesion. Personally, you are just scuffing it and then going to urethane primer for blocking, Id use a red scuff pad which is equal to about 400 grit. A red scuff pad just roughs up the surface so paint will stick.
By the way, most filler primers IF they are urethane based can be used as sealers if the mix ratio is changed. Look at the product tech sheets for the primer you have. Personally, I use sealers...Eric
