On Apr 7, 10:12*pm, " wrote:
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.

Good post Mark, I agree with your observations. When my trailer is
left outside, I open it every month and move wings and fuselage away
from padded areas..............best way to do this is to assemble the
bird and take a tow. One area where PU is superior to PE is it resists
cracking where gelcoat doesn't. My Nimbus 3 came down from a wave
flight with hundreds of cord-wise cracks and I deliberately came down
slow and avoided the rotor. My Genesis has PU and I'm quite happy with
it, but it's stored in the barn all winter long.
JJ