Typically filler needs a coarser grit under it. After epoxy most people spray a 2k primer or high build primer which is much easier to sand than epoxy. Block it 180, reprime and then sand 320/400 then 600 depending on whether your going to paint or sealer.
TOTALLY what he said. You will regret sanding with 400 and then applying filler. 400 will not give the 'tooth' required to give the filler something to really stick to.