Wing Bubbles and re-surfacing

I've got the same problem Steve does with my Apis. This is a brand new
glider with a brand new Prestec finish. We sprayed it on during the
winter of 2002, and by the winter of 2003 it had developed a nasty
rash of bubbles all along the LE of the wings and all over the
stabilizer. They mostly dissapeared the following summer, but are
starting to reappear now that the winter is here.

Oh well.......sigh.......!

Brad
199AP





"F.L. Whiteley" wrote in message ...
If Steve normally keeps his DG in Western Washington, be aware that average
annual AM humidity is around 83% and average annual PM humidity is around
62% around Puget Sound. GRP will also accept moisture via osmosis and
there's no gel or moisture barrier inside. Long term I suspect gliders may
achieve some sort of equilibrium with their surroundings WRT to moisture
content. It would be interesting if someone has done an accurate W&B on the
same glider that's been at one of the more arid sites (=10% average
relative humidity) for say 5 years, then moved to somewhere like western
Washington for 5 years and noted if there's any measurable change.
Depending on the airframe size and contruction type, I wouldn't be surprised
to see variances of 2-10lbs due to moisture content. I found a moisture
detection meter sites that indicate that 3% (not clear whether by mass or
volume) is high moisture content in GRP substrate. GRP type products
typically have a SG range of 1.2-1.9.

FWIW, I've seen a couple of gliders exhibit small localized bumps from
soaking wet trailer pads that vanished within 24 hours when dried, so unless
there's obvious permanent deformation or checking, it may not mean there's
debonding. I'm also aware of gliders that have had the gel severely damaged
by cold soaking followed thermal shock in a humid locale. I suppose if the
moisture goes through rapid phase state changes it could be damaging. I
found this interesting http://www.cerf.org/pdfs/collab/durability/3.pdf

Frank Whiteley

"Mark Zivley" wrote in message
. com...
The Gelcoat itself is slightly hygroscopic (sp?) and will therefore
absorb and retain some amount of water. If you refinish with Urethane
that issue should go away and you shouldn't need to worry about moisture
permeating the composite structure.

Good luck,

Mark

Steve Hill wrote:
After my nice flight last month in Mt. Rainier wave to 24k, my wings
have
turned into a blistered nightmare on the DG-400. I probably have 2,000
small
bubbles ranging from .005 to .015" tall...I've been sanding the gelcoat
down
in an effort to see if I can break through them, but I am nervous as
hell if
what I'm doing is a bad idea...Is there any chemical that will remove
Gelcoat and not hurt the substrate, or is the best method to actually
just
sand till ya can't sand no more???


Anyway...my thought is to sand the wings down, but then I'm not really
clear
on what's going to keep the moisture from returning for good...it seems
like
baking the wings is a decent idea, though for now I'm not exactly sure
how
and at what temperature...Anyone's input will be appreciated...again,
the
worry is that if you go to all the trouble to sand things down to bare
material, is there a way to assure that the bubbles don't come back in
the
future. My plan would be to re-finish with one of the PolyUrethane
systems...So, time, temp, advice or chemicals!!!!

Thanks


Steve.

I have had a glider that when it got below 32 it developed goose bumps all over
the wings. This was during the early 80's. The factory sent over folks to see
this after they saw the pictures. It was found that when they sprayed the gel
coat into the wing molds that mosture was present in the air compressure and
the filtering system they were using was not getting all the mosture out. They
took the glider back, and I got a brand new one. One of the few times that the
factory stood up for its product. Thermal tight, Soar high, Fly safe, # 711.