: Stupid Me

Not even. There are more brains here than I have seen anywhere else. What are you trying to do? Full on revarnish, or patching? Someone with more experience feel free to chime in, but I think that what you want to do is give the new coat some tooth to grab on to. If it's varnish, just sand till smooth and recoat. If you are repairing or laying new goo on, use your hands to tell you when you have sanded enough. With a coarser grit like 120 or 150 you should be able to differentiate between epoxy and varnish. They will "feel" different. Sand until you have a uniform surface and repair/recoat.
Disclaimer: I haven't refinished a boat yet, but this is what I would do.

Larry