I believe the blisters form when water collects between the substrate
(epoxy+glass structure) and the surface coat. The water vapor
pressure increases when the temp rises and forms these
"blisters" (bubbles) as it debonds the surface coat from the
substrate. Gel coat and polyester paints (Sim/Prestec, polylux,
Ferro) are porous and epoxy resins are hydroscopic. The "perfect
storm" for blisters. Water infiltrates from the inside - out and
forms blisters even when PU and other nonporous coatings are used.
The most widespread and largest blisters I have ever seen were on a PU
coated ship that was stored wet in a hot, humid, leaky trailer in the
southeast; baseball to basketball sized. The problem is even worse
is there is primer/surfacer between the structure and the surface coat
as it is very porous and absorbs and holds water very effectively.

The physical characteristics and composition of trailer padding has a
large impact on blister formation. The wool felt padding used in old-
school trailers is far superior as it wicks moisture away from the
glider and allows it to dry. Synthetics hold the moisture up against
the surface and allow drying only very slowly. The padding used in
modern Komet and Cobras is especially bad due to the rubbery backing
(you know, the stuff that turns to powder in a few years of the desert
southwest and is amazingly difficult to remove). Sadly, wool felt
is very expensive but worth it in the long run. Many 40 year old
Eberle trailers still have their original felt padding that is in very
good condition. This in contrast to the Komets and Cobras where
scraps of rotted "carpet" tatters flap in the wind.

I vote polyester for an R&D ship or a racing ship that will be
periodically re-contoured and for those that do not have access to a
real paint booth and a fresh air, positive pressure breathing system.
PU is very toxic due to isocyanate catalyzation. They are NOT for
amateur use.

The exposure and wear superiority of PU over Simtec, Prestec, Polylux,
and Ferro are vastly overstated, IMO. Note the 40 yr old Libelles,
Cirri, and AS-W15's with their good condition gel coat. The original
Simtec/Prestec formulation had a mil-spec number on the can. Finally,
polyesters are also much less expensive and far easier to touch up and
repair. And they will not poison (kill / permanently injure) you if
you take even the most basic precautions.