another gelcoat story

[Brad[_2_]]

When we made my fuselage, it was winter and we were heating the shop
with a propane heater. Propane releases moisture during combustion.

So........molds are waxed multiple times, gelcoat (shurfab polyester
product) is sprayed into the mold, we leave the heater on and go home
for the night............gelcoat cures.

Next day come back to the shop, make up a batch of epoxy and a bit of
micro as the first layer against the gelcoat and then start laying in
the carbon, no veil cloth, the idea was the epoxy/micro slurry would
help reduce print thru.

Fast forward 2 years.............fuselage is now in my shop where I
have been working on the interior bits this spring/summer. I am bored
and decide to sand the fuselage with 220 then down to 320 to get ready
for top coating with Prestec. Wow.............it looks great.

I blow off the dust with a compressor and then wipe the fuselage with
a damp rag and go watch TV for a bit. A few hours later I go into the
shop and am horrified to see large areas of blisters have formed under
the gelcoat...........I am able to peel the gelcoat off in places with
my fingernail! Further investigation with a sanding block and 70 grit
paper confirm there is no secure bond between the gelcoat and the
epxoy slurry in enough areas that I resign myself to remove the
gelcoat off the entire fuselage, down to the carbon.

1/2 is already done and I'll start today on the other
half...................ugh........

so.....my theory: the gelcoat had moisture on it when we started
brushing in the epoxy...........and probably there was a temperature
issue as well that may have prevented a full cure of the epoxy/gelcoat
interface.

Anyways.............a lot of work for sure.

Next step...................get some Prestec 2481 and brush it on and
start sanding again. Gotta do this before the WX turns here in
Washington and we start the rainy season.

Brad

[Martin Gregorie[_4_]]

On Sun, 24 Aug 2008 09:28:06 -0700, Brad wrote:

> When we made my fuselage, it was winter and we were heating the shop
> with a propane heater. Propane releases moisture during combustion.
>
Yes, quite a lot of moisture. A back of the envelope calculation says
that every pound of propane burnt produces just over 1.6 pounds of water,
so you get an imperial gallon of water from 6 1/4 pounds of propane.

Have you any idea how much propane you might have burnt that night?

I'd put money on the water being the problem. IME the worst low
temperature can do to good, well mixed epoxy is to lengthen the curing
time. We used to say that Araldite glue must be cured a bit above the
maximum temperature the item will be used at or it would soften in use,
but I don't know if that's a general rule or specific to that formulation.

Bad luck though. I can't think of many more irritating ways to find out
about the bonding problem.


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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