The closer you get to a bare epoxy bottom, the better the paint job will look and the more impressed the fish will be. Sand until you find yourself wondering just why you're trying to impress the fish. At least sand off all the loose paint and dirt. Any paint that's left on, sand the edges to feather them so you won't have obvious paint plateaus.

Unfortunately, there's no real shortcuts.

Have fun,

Laszlo

   Thank you L. My sense is that a vibrating sander will be a better choice than, for example, a disc sander. My goal is to eliminate the really nasty spots and rough the existing paint to prepare the bottom for a new coat. No desire to take it down to bare wood.... thank you again and happy sailing     Tomc