Whatever you do, you cannot completely protect fiberglass. Whtever you apply will cause water to penetrte to it. Gellcoat is permeable to some extent, Epoxy is better but not waterproof. For blistering not only water but some chemicals are also needed. They are generally present in the plastic of the hull. It depends on their quantity to bond with water and form blisters. That means osmosis is not the real reasonof blisters but the chemica reaction with water and the chemicals inside the fiberglass.
The amount of water in the hull is not as much as it is thought. A dry hull has a maximum of 1 - 1,2 % water in its content. The maximum is something like 3%, which means the boat will be heavier maximum 3% of its hull weight.