The reason you do not want to use polyester resins on an epoxy board with an EPS (Expanded Polystyrene) core is that the polyester resin, or the catalyst/hardener, will
melt the core foam (EPS) in your board.
Epoxy resins will not "react" with EPS foams, so epoxy based products are the way to go.
If you just have a little scratch in the paint, just get some paint that matches the color of your board (in a spray can preferably, but you can brush on regular paint also).
Mask off the adjacent undamaged areas, spray a couple of thin coats of color matched paint on the scratch, let the paint dry.
Remove the masking tape and sand the area with progressively finer abrasive cloth and you will get a professional looking repair.
If the structural shell (the sandiwch) of your board is damaged, then you need to remove the damaged area, fair in the adjacent undamaged area, and use West Systems or System 3 epoxy resin and some fiberglass cloth to restore the structure in the damaged area.
Then mask off and paint your repair and your board will look good and be almost as strong as if it were never damaged.
Check The Board Lady.com website for lots of excellent information on how to repair your board.
Here's the link:
If you need to fill in a scratch that's a little deeper, but not through the layers of fiberglass, hard foam and wood, you could safely almost any "filler".
Marine Tex is a good one as is JB Weld and JB Weld Quick.
Hope this helps,