Thanks for all the input! We're finally on the water again after a lot of sanding, fiberglassing, epoxy, and painting. Ended up repainting the whole bottom of both hulls with two-part polyurethane paint after fiberglassing the damaged areas, the poly should be stronger and more resilient compared to gelcoat.

The sad part of the story is that we still have a leak-problem. Been on the water a couple of times, it's not as bad as before, but a couple of liters of water inside the hull after an hour or so. Really annoying problem, think we need to bring soap and blower to the beach to check for cracks again. I suspect the daggerboard case, this is the only part not painted with polyurethane. We only epoxied the cracks many times and crossed our fingers. Daggerboard fit is tighter, but still easy to get in and out, should be possible to epoxy some more.