Fiberglass boats can be damaged by water intrusion through the gelcoat, or water intrusion into the sandwich core, for example from a through hull fitting which is placed in the core, not solid fiberglass. Water intrusion, nowadays, through the gelcoat is much less common than 15 years ago as water resistance has been improved with different gelcoat formulation. Moisture meters detect resistance, dry fiberglass has high resistance, wet fiberglass has low resistance. Dry fiberglass is between 0 and 12, Moist is between 15 and 22, wet is above 22 with more than 30 indicating water is present. Below 22 we recommend that the situation is monitored. Above 22 we recommend destructive testing. Wet core from a badly fitted through hull or a crack (which could be above the waterline) can be a significant repair cost. Water intrusion through the gelcoat normally requires pealing the gelcoat and then time to dry ( often 3 or 4 months) before refinishing. Wet core, if Balsa core, the wet balsa will need the core removed and replaced, If foam, it may be possible to remove the moisture by using a strong vacuum. The hull bottom must be dry before any evaluation, we recommend at least 3 days dry storage before the test. The meter can provide misleading results, for example – carbon fiber is conductive so will show high readings, some fillers are conductive and will show high readings. We use a Tramex Skipper + meter which is designed for fiberglass. There are less expensive meters designed for domestic use which should never be used on fiberglass.