On 11/15/2015 7:47 AM, son_of_flubber wrote:
On Sunday, November 15, 2015 at 8:57:36 AM UTC-5, wrote:
For other manufacturers using T35 and similar it is mostly UV and
moisture that causes the degradation of the surface.

I heard that the cracks develop in the direction of the final sanding
(front to back). My understanding is that the sanding leaves microscopic
cracks in the direction of the sanding, then moisture gets into the cracks
and expands them. And the PU coating seals those microcracks and slows
down the penetration of moisture and the subsequent gradual widening of the
cracks?

When I apply Occam's razor to gel-coated, commposite gliders, I see relatively
flexible structure beneath relatively brittle gel coat, the latter which -
unsurprisingly, to me - tends to crack. Think chocolate-covered licorice
twists (or some such) - which material's gonna crack first?

The thicker the gelcoat, the more bending strain through its depth, and the
more likely a place for cracking to appear, but I wouldn't bet my life bending
and stiffness are the only contributors. In any event, polyurethane paint now
has a long history of being far more crack-resistant in glider use than every
gel-coat employed to date.

Some years ago, my club sanded one of our G-103's wing-pairs down to the
bottom of its very many (top & bottom/"classic pattern") cracks (because a
glass-inexperienced-annual-guy refused to sign off on it), then had an auto
shop shoot polyurethane. In some places we went down to the glass; in others
we did not. No cracks *into* the glass were found. In the 4 years or so before
we sold the - regularly used in wave - ship, nary a crack reappeared on any
painted surface.

Bob - chief sanding grunt - W.