Gelcoat Crack Fix
There are a lot of reasons why spider cracks form. Some are pretty benign but others represent a much more serious problem. If I had a 15 year old boat with a lot of spider cracking I would want to know why it had occurred. As to fixing them, I know of no way to make repairs to a spider cracks that leaves the original gelcoat intact and visible. The usuual cure is to grind down through the original gelcoat to the substrate, look for cracking there and if that is solid, either paint or gelcoat the area back in. This is next to imposible to do seamlessly as a spot repair in a non-skip area.