I was actually trying to avoid this part of the discussion, but here goes…..
Interestingly enough, boat hull blistering is not a gel coat problem, but
rather a laminate problem. Considering the laminate as a semi-permeable
membrane the potential for osmosis can take place. Osmosis is the tendency of a
fluid of lower concentration to pass through a semi-permeable membrane into a
solution of higher concentration. In the case of boat hulls, water vapor (lower
concentration) passes from the inside of the hull to the outside of the hull
(higher concentration). The gel coat matrix is denser than the laminate matrix
and the transmitted water vapor will eventually collect in what are known as
seed sites. These are voids at the gel coat/laminate interface. Eventually, the
liquid in the seed sites will become denser that the outside water and the
process reverses pulling water in from the opposite direction…. This is where
the big nasty boat hull blisters appear. Blistering problems have been solved
by the boat or swimming pool industries, because they now use vinyl ester skin
coats behind the gel coat. The point is that the problem was solved with a
laminate modification, rather than a gel coat modification.

The reason I was trying to avoid this discussion, is that this mechanism is not
in play in relation to gel coat cracks. Sailplane gel coat, or more likely
urethane paint, can blister from osmosis. Just put a glider in a wet fuselage
cradle! However, until the seed site is saturated and dense there is no
transport of *liquid water*. All the moving H20 is vapor phase. This only
happens under very specific conditions. So unless you are going to ride it
hard and put it away wet there is absolutely issue with washing a gel coat
finish. But even when blistering takes place, there is no relation to typical
gel coat cracking.

Bob Lacovara