Should I strip an unknown old finish before applying epoxy?

If you don't know what the finish is, the best option is to sand back down to raw wood and start from there. Without knowing the coating, we can't say whether the epoxy will be compatible with it.

If you restain the bare wood, use an alcohol- or water-based stain. They dry fast, and UltraClear Bar & Table Top Epoxy can go over them in about 24 hours. Oil-based stains take a long time to fully dry; if you use one, apply 3 coats of oil-based polyurethane over it before the epoxy seal coat. Epoxy applied directly over an oil-based stain that isn't completely dry may not stick and can peel off months later.

If stripping isn't practical (a set of restaurant tables, for example), you have two options. Apply 3 coats of oil-based polyurethane over the old finish to create a layer the epoxy will bond to, or test first: clean the surface thoroughly with acetone and clean, lint-free rags, coat one table or a small area, and confirm the epoxy adheres before doing the rest. Either way the surface must be completely clean and free of dirt, dust, and oils, and you still need the epoxy seal coat before the flood coat.

If you know anything about the existing finish, send us the details and we'll tell you what prep it needs.