If you have fiberglass that is cracking you should take a drill bit and drill a tiny hole at each end of the crack which will prevent the crack from growing any further than those holes.
Then clean the area really good and get yourself some epoxy resin or also known as a fiberglass repair kit and then apply that fiberglass resin to the crack and the holes you drilled and allow it to set up and cure.
The crack should be fully repaired after the fiberglass epoxy resin cures and you should no longer get any further cracks unless something else causes another crack in the fiberglass.
If you do get more cracks then just drill a few more holes at each end of the crack in the fiberglass and apply some of that epoxy resin again to fix the cracked fiberglass.