Before applying varnish or urethane to epoxy, let the epoxy cure for at least 5-7 days, under normal curing conditions. Then, do the following surface prep:
- Remove any amine blush by washing the cured epoxy surface with a mild soap and warm water. Note that amine blush may or may not be visible, and it can also occur with epoxy products that are non-blushing, so it's always a good idea to wash the surface, to ensure best results.
- Allow the surface to dry completely.
- Sand the surface with 220-grit to 400-grit sandpaper to ensure a strong bond.
- Remove sanding residue.
- Wipe the surface with an approved solvent wipe safe for the varnish or urethane product you're using.
- Let the solvent evaporate completely.
- Apply the marine varnish or urethane, as directed.