Varnish compatibility with different substrates

WOOD

• Bare wood: The wood should have a moisture content of 15-20%. If it's too dry, the varnish finish can check, crack, or lose gloss.
• Previously varnished wood in good condition: Before sanding, remove all surface contamination. If the previous coating isn't cracking or peeling, you don't have to remove it. The exception is if you're applying a two-part varnish over a single-part spar varnish or urethane, In that case you have to remove the single-part varnish first.
• Previously varnished wood in bad condition: In this case, remove the existing varnish with a heat gun or chemical stripper and start from bare wood. A heat gun is especially effective on irregular or curved areas. Be careful not to burn the wood or damage fiberglass, hatch composites or portlights with the heat gun.
• Wood that's been stained: The stain has to be completely dry before you apply varnish. To ensure that you get the desired results, test first by applying varnish to a small scrap of the stained wood.

EPOXY

• Epoxy must fully cured 5 to 7 days, depending on the product and the temperature. 
• Wash the cured epoxy surface with water and scrub lightly with an abrasive pad to remove any amine blush - even if you can't see it. Let the surface dry completely.
• Sand to smooth out any runs or rough spots. Start with 80-grit sandpaper, then 150 grit, and end with 220 grit.
• Vacuum to remove all sanding residue, then clean well by wiping with a clean, lint-free cotton rag dampened with acetone or denatured alcohol.

POLYURETHANE

• Generally, it's not recommended to apply a solvent-based spar varnish over polyurethane. However, you can apply a polyurethane finish over a spar varnish finish that's in good condition.