Ive read you have to drill a hole at the end of the crack to stop it from spreading. And then you can use a fiberglass kit to fix it.
What i did was scuffed the areas and applied jb weld and then duraglass filler over top after i sanded the jb weld.
I applied the jb weld on the other side of the cracked area. You have to scuff the area befoee applying it.