Can I apply epoxy over spar urethane, marine varnish, or other varnish?

Not directly over varnish. UltraClear Bar & Table Top Epoxy doesn't adhere to varnish, so we don't recommend pouring it straight over marine varnish, spar varnish, or a light varnish finish. Do one of the following first:

1. Seal the varnish with oil-based polyurethane. Apply 3 coats of an oil-based poly over the varnished surface and let it dry, typically 1 to 2 days. Epoxy adheres perfectly well to oil-based poly, so the poly forms a bonding layer between the varnish and the epoxy. Then lightly sand with 220–320 grit, clean with acetone, and apply the epoxy seal coat and flood coat as normal. Varnish doesn't count as a seal coat; you still need the epoxy seal coat before the flood coat.

2. Or test first. If you'd rather not add poly, coat a small or hidden area and confirm the epoxy bonds before committing to the whole surface.

Spar urethane is a separate case. We've recommended the epoxy for a bar top finished with three brushed coats of oil-based spar urethane: the flood coat self-levels to 1/8" and cures to a smooth, glass-like finish, so brush marks disappear under it. If you're not sure whether your finish is a varnish or an oil-based urethane, test a small area or add the 3 coats of oil-based poly to be safe.

If you're starting from bare wood and want to stain, alcohol- or water-based stains can be epoxied over directly; an oil-based stain needs the same 3 coats of oil-based poly.