Failed gelcoat absolutely "prints" down into the glass/epoxy substrate. We've just refinished two gliders in our club which were left to go beyond minor crazing to full-on gelcoat failure. Once the gelcoat is totally stripped, you're left with visible lines in the glass/epoxy. Under a 4x loupe you can see that there is an edge to these lines where there are small voids in the epoxy.

Does this materially affect the integrity of the wing/fuselage/control surface structure? That's beyond my pay grade.

Gerhard Weibel's take is as follows (from the Schleicher Technical Note on "paint cracks"):
Owing to the UV-radiation the gel coat of the paint surfaces grows
brittle and shrinks; at the same time the UV-light destroys paint ingredients.
So moisture (rain, dew) working in on long term will wash the
decomposed paint ingredients out off the paint. The paint starts chalking
and gets hairline cracks owing to the concurrence of embrittlement
and shrinkage. Furthermore, these hairline cracks gather dirt
which through its aggressive effect and its stronger heating-up from
sun radiation further precipitates the degradation of the paint. Owing
to this the intended protective effect for the fiber composite structure
against moisture and UV-radiation is no longer granted.

Certainly a good care with hard wax can slow down the above process
distinctly, but it cannot be stopped completely. For this reason a
repainting of the aircraft will always become necessary at some point
of time.
However, we point out explicitly that paint cracks - even deep cracks -
do not represent damages to the aircraft structure if as of their first
appearance immediate correct maintenance and care is given furthermore
to the aircraft.
As all the outside skin of the aircraft is dimensioned for stiffness, there
are no critical mechanical strength problems, even if some cracks
have gone down into the fiber composite structure and have already
attacked the resin matrix base.
The unknown ageing effects caused by the influence of moisture and
UV on the unprotected fiber composite structure are more dangerous.

P3

On Tuesday, August 30, 2016 at 7:20:26 PM UTC-4, Bob Whelan wrote:
On 8/30/2016 2:42 PM, It was written:
Aerodynamics isn't the issue - gelcoat failure can progress until it's a
structural issue.

In the spirit of seeking genuine knowledge - and readily acknowledging
widespread *concern* that cracking/flaking gelcoat can "somehow" morph into a
"premature" structural issue - can anyone cite solid information (data, links,
etc.) shedding light on the mechanism(s?), location and quantity of UV-exposed
plastic sailplanes thrown onto the garbage heap after failing structural
tests, etc.? It's a serious - not rhetorical - query.

There's plenty of engineering data, as well as "common-sense/observational
data", around indicating UV is a catholic attacker/degrader of all manner of
materials, but have sailplane manufacturers, or (say) the LBA, or anyone else,
set about obtaining such data for GRP/CRP sailplanes?

Hard data eagerly welcomed!

Bob W.