Refinishing: Who has tried a shortcut?

Brian Case wrote:

We have a Pegasus here that was resprayed over the old gel coat. I
don't know much about how it was done as it was not done locally. What
I can tell you is that nearly every single one of the old cracks in
the gelcoat are now showing through the new finish.

Do you know what was used for the new finish? Is it gel-coat or 2 part
polyurathane paint? If it is paint, what kind of primer went on underneath
it?

Ian

Do you know what was used for the new finish? Is it gel-coat or 2 part
polyurathane paint? If it is paint, what kind of primer went on underneath
it?

Ian

It is not a 2 part polyurathane paint. I suspect it is Gel Coat on top
but I can't say for sure.

Brian

I have refinished an old Mosquito that had badly cracked
wings. I did it with Glasurit 2K and high build epoxy
primer.

We took off enough old gelcoat that the surface looked
good, we did not remove all old gelcoat.

We took lots of time removing grease/oil/dirt from
cracks in order that the paint surface would not be
damaged by grease causing the paints not to stick well.
Used lots of degreasing agent, acetone I think.

The epoxy primer took very well and was sanded back
for smoothness then painted with the top coat 2 pack.

I haven't seen it for a couple of years but last time
I did the paint was still holding up and not showing
any signs of crack propagation through the primer and
top coat.

When it was finished, I was very pleased with the results
and the glider flew much better having got back a great
portion of its laminar flow.

The gelcoat, although cracked, was NOT peeling off.
It still was a suitable surface to paint on.

John

If old gelcoat is left behind, would not it eventually
crack and propagate to the surface? Also, and I have
googled RAS on this subject, is UV penetrating through
the cracks to the structure harmful? IE, will refinishing
a ship as soon as cracks appear save time and money
in the long run?

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

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

Greg,
Because most sailpanes are waxed throughout the year.
Reuben
Greg Arnold wrote in message news:BdREb.20434$J77.19108@fed1read07...
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

Good report, Bob
I would only add one data point. You saidThere must always be
concern about an underlying
(but now covered) crack propagating into the laminate.

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

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

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