Gluvit is an epoxy, and while I haven't used it, I have used epoxy a bit and read up on its use because of various boat-building projects.

My gut feel is that you can just patch the spots, assuming you prepare it adequately. I'd guess you could get a couple of years out of it that way. However, if you treat the entire bottom, you'll get better service, assuming that the original coating isn't peeling off.

Whichever you choose, you'll need to remove the loose flaking Gluvit and sand everything. The sanding doesn't have to be to bare metal. What you're trying to do is clean off the impurities (an acetone wash might help here, but I'd use whatever is supposed to thin the Gluvit) and then sanding to provide "tooth" or grip for the new epoxy.

Good luck,

Keith