A proper cure for cracks ain't easy. I bought a new Catalina 309 in 2007, and it developed many long gelcoat cracks all over the deck after the first Michigan sailing season. A lot of back and forth with Frank Butler at Catalina finally resulted in them sending a guy out to fix the cracks.
I watched him do some of the repair. He started by using a Dremel tool with a grinder bit on it and widened each crack to about 3/16", and went right down to the glass fibers. Basically, huge trenches. He filled those with a mixture of glass and thickened epoxy, and topped it off by spraying on gelcoat. Followed by sanding with various grits of sandpaper, and using ink as a visual indicator to determine when the repair was flush with the surrounding area. Where the cracks went through the waffle non-skid pattern, he resculpted the ridges by eye/hand. When he was done, you couldn't tell where the cracks had been.
Don't know if there's a moral to this story, unless it's to seriously consider just keeping the cracks clean as some have suggested.
Alternatively, I restored an old Grampian 26 that had a lot of gelcoat cracks and other damage to the decks. I widened the cracks, filled them with thickened epoxy, sanded smooth, and covered the entire deck with two part epoxy paint.
There is no easy cure for gelcoat cracks.
