Refinishing: Who has tried a shortcut?

I have worked on several gliders over the past 25 years where the =
gelcoat cracks did indeed propagate down into the glass structure. One =
in particular was an ASW-15 where we had to zip off the entire outer =
layer of unidirectional glass in order to remove all of the cracks. =
These were crazing cracks over the entire surface, not stress cracks =
from load (crashing,etc.). A severely cracked LS-4 comes to mind as =
well. With good light and a jeweler's loop, we have noted a good =
handful of other ships that had localized crazing that had gone down =
into the structure. Speaking of down under, the Australians have noted =
this problem and issued an "AD" (or whatever they call it) concerning =
this.

Paul

One of the main reasons our gelcoat gliders crack up is because of the =
final finishing process. Boats are popped out of a shiny mold and are =
basically done. Glider pieces that are gelcoated get additional sanding =
for contouring, etc.. This breaks up the surface gelcoat which, at the =
time of process, has additional "protective" cross-linking at the =
surface. This is great for contouring, but a no-no for finish =
longevity...unless you immediately paint over this surface with poly =
paint, etc..

I have noted several gliders that were refinished with poly paint where =
all of the old crazing was not removed. One $10K, 2-seater refinish job =
has cracks returning after 3 years.

Paul=20

Ian,

I have had excellent results with a 'partial' gelcoat removal/refinish ...
repainted with Simtek Prestec ... fuse 5 years old, still perfiect ... as
are wings 3 and 2 years since refinish. This is a lot of work, BTW.
Documented on SRA site ...

http://sailplane-racing.org/Articles...asw20_wing.htm

BTW ... did I say this is a lot of work. JJ ... apologize for stealing your
punch line. :-)

KK
Ken Kochanski


"Ian Forbes" wrote in message
news
Time for a round up.

I started this thread 4 days ago and I asked:

There has been quite a bit on refinishing on RAS recently. Most people
advocate that when gel-coat starts cracking up, it is essential to
remove
it ALL.

There has been lots of advice and theory as to why the above is true. No
doubt it is the recommended approach - recommended by many who are not in
a
position where they might have to pay for the work!

However there was little response to my question:

Does anybody out their own a ship which was refinished without removing
all the gel-coat? What is the service history of these gliders?

So far there have been two postings of problems with re-finishes. Neither
of
these involved a polyurathane finish. Gliders re-finished with extensive
or
complete removal of the old gel-coat have held up - at least as well as
the
original gel coat.

So I will extend the question. Does anybody anybody know of gliders that
have been refinished with polyurathane and subsequently experienced
problems?

I can answer that by saying I know of one glider that was re-finished with
polyurathane about 10 years ago. There is some localised paint cracking on
the tailplane and rudder which suggests that there were some areas of poor
preparation during the refinish. I suspect that the glass skin on these
areas is thiner more flexible which results in a finish that is more
susceptible to cracking. This was not an "expensive" refinish job, for
example the ailerons and flaps were not removed during the respray. But
overall the glider still looks very neat and I think I can say this finish
has lasted as least as well as many factory gel coat finishes.


Ian

Ian - painting a control surface without removing it means it has not
been checked for proper mass-balance, and for many gliders is likely
way out of spec - flutter hazard and not airworthy. If you refinish
over cracked gelcoat, the cracks will continue to propagate upwards
through the finish and downwards into the laminate - not airworthy
and hidden.

A reputable and knowledgable mechanic will sign off neither of
these "shortcuts".

Be careful out there,
Happy Holidays, Dave

PS: Yes, I have seen gliders where these shortcuts were taken,
and yes, it caused serious problems later.

Ian Forbes wrote in message ...
Time for a round up.

I started this thread 4 days ago and I asked:

There has been quite a bit on refinishing on RAS recently. Most people
advocate that when gel-coat starts cracking up, it is essential to remove
it ALL.

There has been lots of advice and theory as to why the above is true. No
doubt it is the recommended approach - recommended by many who are not in a
position where they might have to pay for the work!

However there was little response to my question:

Does anybody out their own a ship which was refinished without removing
all the gel-coat? What is the service history of these gliders?

So far there have been two postings of problems with re-finishes. Neither of
these involved a polyurathane finish. Gliders re-finished with extensive or
complete removal of the old gel-coat have held up - at least as well as the
original gel coat.

So I will extend the question. Does anybody anybody know of gliders that
have been refinished with polyurathane and subsequently experienced
problems?

I can answer that by saying I know of one glider that was re-finished with
polyurathane about 10 years ago. There is some localised paint cracking on
the tailplane and rudder which suggests that there were some areas of poor
preparation during the refinish. I suspect that the glass skin on these
areas is thiner more flexible which results in a finish that is more
susceptible to cracking. This was not an "expensive" refinish job, for
example the ailerons and flaps were not removed during the respray. But
overall the glider still looks very neat and I think I can say this finish
has lasted as least as well as many factory gel coat finishes.


Ian

Paul,

I agree that this additional surface work could lead to, down the
road problems. I do not think this is because of changes in the
crosslinking, at least not entirely. Crosslinking is the bridging of
the loose ended polymer chains. By designing the polymer of choice (in
this case gelcoat) to undergo crosslinking you create one giant
supermolecule. And during the painting process as long as each
additional coat is applied before the crosslinking is finished, each
additional coat will add to the matrix. But after the fact this
chemical reaction will not take place to the same extent and a
mechanical bond becomes the order of the day, hence the surface
roughing requirement during repairs.

*As a side note: the amount of crosslinking involved plays a large
part in the amount of flexibility a finish will have. By using a flex
additive you can inhibit the crosslinking in some or all of the paint.

My company's work with interfacial chemistry has long shown however,
that like a lot of polymers, even gelcoat is semi-permeable to water.
So, for example when you wash your glider, then dry it and finally wax
it (because it is all clean now, right?), your drying was never really
able to extract the water from the pores. The wax seals it in and
capillary action along with other hydroscopic forces pull (or drive)
the water in deeper where it freezes and expands, pushing against the
sanding scratches which now become stress risers, ultimately leading
to microcracks which propagate to larger cracks. Without the wax the
water is free to go, to and from, as external and internal forces
change usually in relationship to the seasons or prevailing weather
conditions.

Reuben

Paul Gaines wrote in message ...
One of the main reasons our gelcoat gliders crack up is because of the =
final finishing process. Boats are popped out of a shiny mold and are =
basically done. Glider pieces that are gelcoated get additional sanding =
for contouring, etc.. This breaks up the surface gelcoat which, at the =
time of process, has additional "protective" cross-linking at the =
surface. This is great for contouring, but a no-no for finish =
longevity...unless you immediately paint over this surface with poly =
paint, etc..

I have noted several gliders that were refinished with poly paint where =
all of the old crazing was not removed. One $10K, 2-seater refinish job =
has cracks returning after 3 years.

Paul=20

Bob, why do sailplanes (which spend most of their time in the trailer)
have such a problem, while sailboats (which spend most of their time in
the sun and weather) usually have a good gelcoat?



B Lacovara wrote:

The composites industry has many years of experience and good understanding of
gel coat cracking mechanisms. To address a few of the comments:

1) A gel coat fracture must be removed to the extent of its depth; otherwise
the crack propagation mechanism is in play and will eventually affect the
surface again.

2) A thick and/or flexible secondary coating will delay the re-appearance of
cracks, but not the effects. There must always be concern about an underlying
(but now covered) crack propagating into the laminate.

3) UV penetration of the laminate is a non-issue unless the crack is huge. In
that case the crack itself will be a structural issue.

4) To date, there is not a viable method of "filling" cracks. This is a
function of the inherent surface tension of the potential liquid materials. If
one could fill cracks there is a fair chance two micro-cracks would appear on
each side of the original crack and eventually propagate into larger cracks.

5) To date, no one in the composites industry has come up with a workable
shortcut. A quick fix solution would be worth big bucks, with gliders being a
tiny portion of the overall application.

Hope this helps.

Bob Lacovara

Reuben-

Fascinating post, would not waxing increase UV damage?

Oh My, Ruben
It sounds like we have finally found someone who actually knows what they are
talking about. Your post brings up several questions:

for example when you wash your glider, then dry it and finally wax
it (because it is all clean now, right?), your drying was never really
able to extract the water from the pores.

Should we be waxing gelcoat? How about urethane finishes?

I have seen little blisters that form when a sailplans is left in a very wet
environment, like a metal trailer. When these blisters are popped, a little bit
of water is evident under each blister. I have seen this in urethane and also
in gelcoat (Prestec) What is your take on this?




JJ Sinclair

At 16:00 21 December 2003, Jj Sinclair wrote:
Oh My, Ruben
It sounds like we have finally found someone who actually
knows what they are
talking about. Your post brings up several questions:

for example when you wash your glider, then dry it
and finally wax
it (because it is all clean now, right?), your drying
was never really
able to extract the water from the pores.

Should we be waxing gelcoat? How about urethane finishes?

I have seen little blisters that form when a sailplans
is left in a very wet
environment, like a metal trailer. When these blisters
are popped, a little bit
of water is evident under each blister. I have seen
this in urethane and also
in gelcoat (Prestec) What is your take on this?

JJ Sinclair

Ruben's post is really interesting and he makes a very
strong case for water exerting a damaging shear force
between phsically bonded coating layers a refinished
glider and for exacerbating cracking - but is it the
whole story for gliders with their original finish?
I think Bob Lacovara also knows what he is talking
about - see his posting of 04-42 on 6th December and
also his article (and qualifications) at:

http://www.wingsandwheels.com/Weathe...ock%20Tech.htm

I will take a lot of convincing not to cover my glider
with a UV protectant wax and, preferably, to have a
PU finish to reduce the risk of surface crazing from
UV degredation.

Having suffered badly from water filled blisters on
one glider I am very conscious of the effects of moisture
but it seems, from what is written about experiences
in the boating world, moisture will get in through
the composite structure to the gelcoat from the inside
out in wet conditions. The boating people say that
the water dissolves chemicals out of the resin on its
way through and that the water in the blisters tastes
chemical but when I tried it on water from our Discus
fuselage blisters it was pretty tasteless.

For crazing I think that UV surface damage top old
gelcoat is the main enemy and water can then start
to penetrate from the outside. Meantime, in damp conditions,
water has been penetrating from inside from new and
this can cause blisters even in gliders with less porous
coatings

So my plan is to get a PU finish, use Wx Seal/Block
during really dry spells of weather to keep the external
surface as impervious to water and UV as possible and
to store the glider in as uv-free dry conditions as
I can achieve.

John Galloway

I would like to beat this dead horse a little more.

I will check and rephrase my "cross-linking" term. I would like B. =
Lacovara to explain the additional protection afforded at the surface of =
a very shiny, smooth de-molded gel-coat part that is not aggressively =
sanded/polished. I do know that in many cases this cracking/crazing =
follows the exact pattern of the sanding strokes/motions that are =
performed at the factories. Note the cord-wise cracking on LS products. =
They used to do much of the sanding in that direction. They also used =
jitter bug type sanders on the back bones of the fuselages. You can =
observe these "eyelash" shaped cracks all down the surface of the spines =
of many LS gliders. Schleicher's wing control surfaces crack =
span-wise...the direction of sanding. Final grit numbers and direction =
of sanding and follow-up polishing can make a huge difference in surface =
longevity too. This was explained to me an a very large repair facility =
in Germany a number of years ago when they demonstrated span-wise final =
sanding in two stages, followed by a chord-wise direction of the polish =
machine. DG employs this technique. Lacovara's paper confirms that =
surface "roughness" allows faster degradation on several levels. You =
can increase the life of your factory gel-coat finish by sanding the =
surface with 1500-2000 grit paper and polishing ACROSS this sanded =
pattern, increasing the gloss. There are a number of polishes suitable =
for this, including the hard stick polishes used by the factories and =
3M's Finesse it and Perfect it systems, just to name a couple. Follow =
this up with WX seal/block system, and you are well on your way to a =
happier gel-coat surface. =20

Dry out your ship, slick it up, use WX block system once, preferably =
twice a year, NEVER tie it out, and store it in your trailer inside a =
basement or hanger, etc..

P. Gaines=20