I concur with Uncle Hank’s assessment. After grinding off all gelcoat, I have seen what looks like a ghost of the crack that was there, but close examination with strong light and magnification, showed some resin degradation in the outside layer, but I could not see broken fibers. That said, I believe deep cracks should be ground out, filled, contoured and re-finished.
Refinishing the whole ship is the hardest work I have ever done, my hat’s off to anyone willing to take on that nasty job!
JJ