The previous owner let my boat sit on a marsh, so my bottom is a little beat up. No pun intended. But he also went over it with some sort of resin. The surface resin has since cracked. He painted it on with a brush and it seems to be a light coat. Is my best bet to sand it all off and replace it with a strip of glass cloth?
If it's resin over gelcoat... it should come off, if it's polyester resin over worn glass then it was done poorly, when done right... the new layers should be chemically fused to the lower layers. Even if it weren't some sort of mystery resin, you should remove material that is cracked and flaking off until you get to some solid or more secure material. There are a bunch of threads that will agree, do the prep work right! Mixing some resin and slapping it isn't the hard part of fiberglass, it's preparing it and finishing it that most DIYs botch.