I think it usually takes a cpl of yrs but it does happen.None of mine have cracked yet either but ive seen many older hulls and they all seem to have em after been bashed arround for a cpl of seasons,small spider cracks etc.Gelcoat is quite brittle by nature,also adding on top of a hull that has cured id think theres a stronger possibility of it cracking and shelling off?.I personally wouldnt glass over it id remove it where needed to install the rails.I agree about the too thick gelcoat cracking ive seen it.
Mart