J-30's came from the factory with gelcoated hulls. However, by now many of them have had at least their top sides painted, especially if they were originally a color other than white. There is a big difference in the repair approach if the "cracks" that you have are only in the gelcoat or if they extend into the hull or deck laminate. Also, if only in the gelcoat they may be from two different causes. One cause is stess on the hull, often from rig tension and not much of a problem, if not below the waterline. These may well recur if repaired. Based upon my observation gelcoat cracks may also be caused by wet balsa core freezing in the winter time and expanding and causing the brittle gelcoat to crack. For example I have seen several J-30's with gelcoat cracks around the exhaust outlet, apparently caused by freezing/thawing and expanding/contracting core. (Water gets in around the exhaust outlet because the outlet was either not properly sealed at the factory or the bedding has not been maintained by the owner, or both.) If only in the the gelcoat and caused by rig stress,gelcoat crazing can be probably be dealt with using a gelcoat repair kit. If deeper, or if caused by other factors, it is a different ball game and I would suggest having a professional fiberglass person inspect and advise you.
