GRP hulls will start to absorb moisure through the gelcoat as soon as the vessel is launched and this will gradually increase the longer the vessel is afloat. The moisture penetration causes very little damage as it passes slowly through the Gelcoat barrier. This is why it is recommended you store your vessel on land at the end of a season to allow the moisture that is still working it’s way through the Gelcoat to evaporate before it becomes trapped behind the Gelcoat and blisters begins to form.
A GRP boat hull is usually a mixture of polyester resin reinforced with glass fibres. This is due to more cost affective build prices, in a process where layers are placed on top of one-another which forms the laminate. This laminate will have very small voids, air pockets within the resin and fibreglass.