Clean it good and rough up the areas to be filled to provide a good surface to bond to. Lay the filler in carefully working out air bubbles and smoothing as best you can. let cure for a few days. block sand to desired finish. Final sanding at 600-800 grit wet should allow th finish to lay on very smooth.
Heres where you need to check if your planning on a heat cured finish coat do a test The temps to cure the finish may affect the epoxy also.