Refinishing: Who has tried a shortcut?

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