What are gel coat blisters?

If in the hull molding process, a void or particle of dirt or whatever was formed under the gel
coat, eventually, water will osmose through the gel coat at that spot.  The water reacts with
the resin and forms glycol and acids, which slowly dissolve the plastics.  The formed molecule
is larger than the water molecule and is now trapped behind the gel coat.  This causes a
swelling (1/2 to several inches) outward of the gel coat like a pimple.  That’s a blister.  As the
blister grows it eventually leaches into the glass cloth and causes delamination below the
water line.  A few blisters is usually not a problem, but large blisters and/or blistering of most
of the wetted surface requires a grinding back of the gelcoat then a long drying process
(months) then an application of a moisture barrier system and reapplication of the anti-fouling
paint.  Definitely desirable in southern climates where boats spend more time in the water and
the 75 to 85 degree water is perfect for blistering. Photo shows open blisters from a 50'
trawler yacht in Florida.
From the Surveyor's Notebook