Harry, good point.

Because polyester gelcoat will absorb water over time people who leave their boats in the water usually apply an antifouling paint which actually sloughs off over time revealing more of the toxic copper or organic compounds to fight off growth. A lot of the racing mono's will apply an epoxy barrier coat to help reduce water absorption under the ablative paint.

Blisters have to be broken and allowed to dry out completely before they can be repaired, faired, and recoated.

I have actually seen some cats with small blisters forming from resting on the carpeted sections of the hull cradles on trailers that had remained damp.