Refinishing: Who has tried a shortcut?

Ian,

My old LS-4 was partially refinished in Prestec about 5 years before I
bought it. It was clearly an Earl Scheib special (for those of you outside
the US - Earl Scheib = cheap auto refinisher of marginal quality). Within
2 years of my purchase, significant checking appeared in several areas.
Inspection with a 4x loop seemed to indicate that the crazing came from the
substrate (old gelcoat) not the outer surface. I think it just reinforces
the conventional wisdom that any areas of loose or badly crazed gelcoat need
to be removed prior to any sort of refinish.


"Ian Forbes" wrote in message
...

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

Very interesting thread. So with moisture being an enemy, what do the gel coat
sages recommend for removing leading edge bugs? Seems that water would be a
violation of gel coat etiquette?

BTW, I'm a fan of WX Seal/Block. It did wonders for my LS-1f and use it 2 times
a year on my -8.

Danny Brotto

(JJ Sinclair) wrote in message ...
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 think for this group, no.

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?

I have seen this. I don't have the one for all answer here, but I do
know that PPG is very quick to caution sanding the primer/filler too
thin, as it can cause lifting to occur. I think that once water finds
this void it will begin to collect with the water's own surface
tension drawing in more. And water can absolutely migrate from the
inside out.
I do think it is a part of the story with factory new finishes as much
as it is for refinishes, repairs, etc. The water really does not care
and all the work is done by humans who have been known to error.

I saw another question asking about UV. And folks, all of this is for
your reading pleasure (or perhaps not), so don't lose sleep.
So, does not waxing increase UV damage compared to a structure waxed
with a UV inhibitive wax? More complex question and looking at the
weekend owner/flier only, I would say yes, but not at a level you
would notice. Why? Because it is cumulative and mechanical damage
would more than likely occur first on a gross level before the effects
of UV were keeping you down. What about the white chalky stuff? Sure,
oxidized, damaged, UV affected topcoat. Buff it off and go fly. How
much UV exposure does your glider see in the box? How much, if left
rigged over night, in the dark? I might be going out on a limb here,
but I say very little. What would the group here say this flier flies
in a calendar year? 50 hours, more? If I chose 50 hours added 25
percent (for time not spent flying, rigging, staged on the line and so
forth) to represent the time of total exposure to the UV for the year.
That is 62.5 half hours for that year, right? At the end of ten years
we would have 625 hours. Not quite a month's worth of continuous UV
exposure. What about 20 years for this owner. 1250 hours. Just shy of
two months of continuous exposure. If your topcoat of choice can't
handle that, Oh boy. What about in your region's winter months when UV
intensity is generally at its lowest level. Hmmm.

Reuben
Interface Sciences Corporation
President / Director of R&D
www.interfacesciences.com
A&P / IA
California teaching credential in 'Plastic and Composite
Manufacturing'





JJ Sinclair

I have monitored sections of gel-coated gliders with and without WX =
block. There is a VERY noticeable difference of yellowing on areas not =
protected. This has been over a 5 year period. As a UV blocker, WX =
block seems to work as advertised.

Paul

JJ - Good point... the distinction is the crack mechanism. The purely cosmetic
cracks (fortunately a majority of glider cracking) are confined within the gel
coat film, and are a product of the performance of the gel coat. More critical
in nature, are cracks resulting from localized laminate fatigue. Many times the
first indication is a gel coat crack *before* the laminate actually shows a
visible crack. This is the situation of concern if the cracks are covered. The
laminate should be thoroughly inspected after the gel coat is removed,
particularly on older, high time or hard use ships.

Bob


I have ground out literally hundreds of cracks in gel-coat that went
right down
to the fiberglass laminate. To date, I haven't seen any gel-coat cracks that
entered the underlying structure. Not saying it can't happen, just haven't seen
it in going on 30 years of smelling fiberglass dust. BTW, I'm talking about
pure gel-coat cracks, not surface cracks that were caused by laminate failure
underneath. That is the first question that a repairman asks, Is this a
gel-coat crack or has the underlying structure moved?
JJ Sinclair

A dead horse with nine lives. My original post was sparked by Greg's
posting about boats and gliders and was written towards that. During
my tenure at Scaled Composites, we never molded a finish (i.e. sprayed
into mold then laid up parts) 1: because of the need to perform
additional external work to the structure 2: hard to control thickness
of coating when using a spray on coating. This variation can distort
fibers in subtle ways possibly changing designed performance
chacteristics of that structure, but also this variation in thickness
in the coating can play havoc on itself. Temperature cycling will
cause this coating to contract and expand. Areas of thicker material
do this more or less than surrounding material of relatively
consistent thickness. This leads to stress which if given a focal
point, over time can materialize as a surface crack or other anomaly.

With gelcoat and other polyester topcoats(or fillers) shrinking about
1% during their cure, one must wait a sufficient amount of time before
sanding. If done to early the finish contracts around the scratches
imposing a force of sufficient magnitude to generate a crack.

Then at Scaled, aircraft where finished in acrylic enamels or
urethanes and today they use primarily urethane, but they still only
sand to 600 grit when they sand. Folks as a crew chief for one of the
high altitude test vehicles I can tell you that it's seven year old
finish (it's age when I left) never showed any ill effects. 66 foot
wing span 65,000 feet designed operating altitude. And we never waxed
any of the aircraft there including this one. I think sanded
structures done incorrectly or cared for improperly will lead to
cracking eventually, but one must objectively identify and observe the
mode of failure. On their own did the sanding scratches exert enough
force to initiate a crack? Or did something else complete the
triangle?


Reuben

Paul Gaines wrote in message ...
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

This thread is what makes ras work. Reuben I have
a 17 year
old ship with some cracking on the wings. From your
experience
with composites, should I go poly or gel? And should
I have all
the old gel taken off, even if this will be more money?

John Galloway wrote:

With respect, you cannot establish good practice by
totting up numbers of for and against contributions
to RAS.

No, but by spending a few hours trolling RAS, I can avoid making an
expensive mistake.

There has been quite a lot of feedback on this topic, and I have also
received a number of private responses by e-mail.

Clearly the situation is not that simple. But from the feedback I think
I can draw a few conclusions:

- The old gel coat should be removed, based on condition, not on
principal. This is a tough call, as no two people will have the same
assessment of any glider. Definitely all loose, flaking or damaged gel
coat must come off. The tough call is how deep does one go to remove
micro cracks. If you go down to glass you have extra work and/or
complications applying the new coating onto the glass surface and
restoring the profile.

- There is some debate over whether or not to use power tools. It seems
JJ's "air file" is the weapon of choice for best results and least
damage to the structure. With power tools it seems possible to remove
close to all of the gel coat if you need to. See Ken's website
referenced below for a good description of the "air file".

- There is some debate as to to the choice of filler to replace the
removed gel coat. Choices are polyester based (gel coat or filler) or
polyurathane based primers.

- I am a still unsure of what is required to restore a proper
aerodynamic profile. The original gel coat gets its shape from the
mould. Fibre and resin is laminated on top to form the structure. How
uniform is the thickness of the factory gel coat? If you remove all or
most of it, then replace it with a layer of filler and sand that filler
to a smooth contour, will the resulting shape match the original
profile close enough to avoid a significant aerodynamic penalty? How
much deviation from the airfoil shape can be tolerated before a
noticeable loss in performance occurs? The commercial shops don't seem
to labour this point. Is it necessary to check the profile with profile
gauges? If so, how far back from the leading edge should one measure?

- For refinishing, polyurathane seems to have clear advantages over gel
coat. There are no obvious negatives to it either. However gel coat is
the choice of purists and it can also be blended into the existing
factory finish for a partial refinish. Gel coat refinishes seem more
susceptible premature failure than polyurathane ones.

- Reapplication is not without problems. Blow holes, silicon
contamination etc cause problems which add time/money to the project.
The other universal problem is sanding through the new coating while
attempting to restore the contour or to remove an imperfection.

- There are a couple of excellent articles by Ken Kochanski on the net:

http://sailplane-racing.org/Articles...asw20_fuse.htm

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

These should be compulsory reading for all of us who own sailplanes with
typical 10 year old finishes. Thanks Ken. Ken used gel coat for his
project but the articles give good incite. Even the writeup on
replacing internal control seals is worth reading.

- It takes at least 300 hours to refinish an entire glider. Add more
time for repairs and modifications etc.

- A good job is one where the mass of material added is no more than the
mass of material removed. However I doubt anybody ever achieves this
objective.

- The factory finish on many gliders delivered over the last 25 years is
a weak point. Many aircraft need re-finishing. While others (Kestrel
19m and Grob have been sighted) seem to last better. Clearly the
factory finishes have evolved around streamlining the manufacturing
process and improving the look of the delivered product. Less attention
has been paid to the longevity of the product. The suggestion that
sanding marks from the factory finish are actually the cause of gel
coat cracks is very plausible. It seems like a Good Idea to get a
polyurathane coat over the gel coat, sooner rather than later. Even DG
have come to this conclusion:

http://www.dg-flugzeugbau.com/pur-lack-e.htm

Thanks to all those who have contributed, both in RAS and in private
e-mail.


Ian

I have stayed out of the debate so far, because I have very
limited experience. I have done three gliders, one of them
a complete refinish, a Cirrus. The Cirrus was done almost
ten years ago and the finish is still like new. The crazing
was extensive . The gel coat was taken off, to or near the
point of making contact with the glas. I maintained the airfoil
shape through out the process, which caused the old gel
coat to be thicker in some areas of the wing than others.
The cracks by then were so fine, they were barely visible.
After that, several stages of reapplying gel coats were done
with sandings in-between. The key was to build and sand till no
more marking of the crazing was visible.

As for maintaining the correct shape of the airfoil, I have a
rule of thumb that it should not exceed 5% overall, but
maintain a very close tolerance when it comes to waviness.
See Dick Johnson's articles on what makes laminar flow.

I have used Duratec and a high quality gel coat, mixed at
1:5 ratio, which gives it a paint like application and finish,
which keeps the orange peel to a minimum.
I know of at least three other applications with Duratec
and the results are still good. I am convinced that the
product, because it was designed for refinishing high temp
moulds, produces a more durable result.

On later projects I used epoxy sanding primer and Urethane.
It is a bit more difficult to sand the finish coat, but the results
are worth it.
Regards
Udo






"Ian Forbes" wrote in message
...
John Galloway wrote:

With respect, you cannot establish good practice by
totting up numbers of for and against contributions
to RAS.

No, but by spending a few hours trolling RAS, I can avoid making an
expensive mistake.

There has been quite a lot of feedback on this topic, and I have also
received a number of private responses by e-mail.

Clearly the situation is not that simple. But from the feedback I think
I can draw a few conclusions:

- The old gel coat should be removed, based on condition, not on
principal. This is a tough call, as no two people will have the same
assessment of any glider. Definitely all loose, flaking or damaged gel
coat must come off. The tough call is how deep does one go to remove
micro cracks. If you go down to glass you have extra work and/or
complications applying the new coating onto the glass surface and
restoring the profile.

- There is some debate over whether or not to use power tools. It seems
JJ's "air file" is the weapon of choice for best results and least
damage to the structure. With power tools it seems possible to remove
close to all of the gel coat if you need to. See Ken's website
referenced below for a good description of the "air file".

- There is some debate as to to the choice of filler to replace the
removed gel coat. Choices are polyester based (gel coat or filler) or
polyurathane based primers.

- I am a still unsure of what is required to restore a proper
aerodynamic profile. The original gel coat gets its shape from the
mould. Fibre and resin is laminated on top to form the structure. How
uniform is the thickness of the factory gel coat? If you remove all or
most of it, then replace it with a layer of filler and sand that filler
to a smooth contour, will the resulting shape match the original
profile close enough to avoid a significant aerodynamic penalty? How
much deviation from the airfoil shape can be tolerated before a
noticeable loss in performance occurs? The commercial shops don't seem
to labour this point. Is it necessary to check the profile with profile
gauges? If so, how far back from the leading edge should one measure?

- For refinishing, polyurathane seems to have clear advantages over gel
coat. There are no obvious negatives to it either. However gel coat is
the choice of purists and it can also be blended into the existing
factory finish for a partial refinish. Gel coat refinishes seem more
susceptible premature failure than polyurathane ones.

- Reapplication is not without problems. Blow holes, silicon
contamination etc cause problems which add time/money to the project.
The other universal problem is sanding through the new coating while
attempting to restore the contour or to remove an imperfection.

- There are a couple of excellent articles by Ken Kochanski on the net:

http://sailplane-racing.org/Articles...asw20_fuse.htm

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

These should be compulsory reading for all of us who own sailplanes with
typical 10 year old finishes. Thanks Ken. Ken used gel coat for his
project but the articles give good incite. Even the writeup on
replacing internal control seals is worth reading.

- It takes at least 300 hours to refinish an entire glider. Add more
time for repairs and modifications etc.

- A good job is one where the mass of material added is no more than the
mass of material removed. However I doubt anybody ever achieves this
objective.

- The factory finish on many gliders delivered over the last 25 years is
a weak point. Many aircraft need re-finishing. While others (Kestrel
19m and Grob have been sighted) seem to last better. Clearly the
factory finishes have evolved around streamlining the manufacturing
process and improving the look of the delivered product. Less attention
has been paid to the longevity of the product. The suggestion that
sanding marks from the factory finish are actually the cause of gel
coat cracks is very plausible. It seems like a Good Idea to get a
polyurathane coat over the gel coat, sooner rather than later. Even DG
have come to this conclusion:

http://www.dg-flugzeugbau.com/pur-lack-e.htm

Thanks to all those who have contributed, both in RAS and in private
e-mail.


Ian

On this thread there have been several references to the lack of problems
with the gelcoat on the Kestrel 19.

The two Kestrels that I owned had no gelcoat cracking up to ten years after
manufacture, although one of them developed shrinkage along the spar line.

In Martin Simons book on Slingsby sailplanes he makes reference to the fact
that George Burton, the managing director, was unable to get the wings on
the Vega produced to the waviness standard of the german gliders then in
production. Presumably this was because the time consuming process of
sanding the wings after removal from the mould would have cost too much.

If this was the case then probably the Kestrel wings also were not sanded ?

Can anyone comment on the longevity of the finish on the Vega ?

Was it the material used on the Kestrel or the possible lack of sanding that
contributed to the lack of cracking problems ?

DB