In a message dated 1/25/2004 8:48:42 AM Eastern Standard Time,
graham@gflight.f9.co.uk writes:
> >I attended the Europa Forum and was very interested in the talk on gel
> >coating. It sounds very interesting, but I just wonder why so few Europas
> >are gel coated.
>
I've been working with a sailplane refinish guru lately and I've learned a
lot about finishing composites. Maybe I can answer your questions.
The Europa wings and fuselage are gel coated, only just enough to fill the
weave of > the cloth, thus reducing the time of filling and finishing for the
> builder. Many composite sailplane manufacturers are getting away from gel coat
> for one simple reason: it always cracks and crazes with time. It's also prone
> to yellowing making it difficult to color match when doing a repair.
It's easier for a manufacturer to produce a gel coated airplane. For a
typical airplane with foam sandwich construction like the Europa the gel coat is
first sprayed into the mold, then the glass cloth is laid in on top, then the
foam core, then the inner cloth skin. Then, it's vacuum bagged and autoclaved.
The result when removed from the mold is a smooth exterior finish that requires
little buffing and finish work. The finish, though, is short lived.
The trend, at least in the sailplane community, is moving towards 2 part
urethane paints rather than gel coat. These paints are usually applied after the
part is removed from the mold and is more labor intensive because it requires
more filling and finishing after the molding process. Some manufacturers are
experimenting with urethanes that can be sprayed into the mold first like
gelcoat, but I don't know of anybody using this in production yet.
The beauty of urethane over gel coat is it's lighter, it remains flexible and
is easily repaired. It simply does not crack with time like gelcoat. It also
provides excellent UV stability for the underlying cloth and resin. Neither
epoxy resin or glass cloth is UV stable and deteriorates rapidly with exposure
to UV. Cracks in gel coat can allow UV to penetrate to the cloth and resin and
if it's not taken care of can lead to structural failure. If you get a scratch
in urethane you can almost always buff it out. Even if you have to re-spray
an area it's very easy to blend with the existing paint because the color is
far more stable than gelcoat.
We recently had a Nimbus sailplane in the shop for a complete fuselage
refinish. The gel coat was yellowed and cracked all over. It looked terrible.
The
gel coat was removed right down to the cloth. We took over 14 lbs of gelcoat off
the fuselage! In some places the gel coat was as much as 3/8" thick! After
the gel coat was removed, non-structural deep surface imperfections were
smoothed out with 3M lightweight cream bondo. (good stuff) Structural
imperfections
were repaired with E-glass and resin according to the manufacturers original
laminate schedule. The surface was then built back up with several coats of
color build primer to fill the weave, then block sanded, then the 2 part
urethane
was applied. After some block sanding with 1200 to remove "orange peel", it
was buffed to a mirror shine. The finish was awesome! As an experiment we let
some catalyzed paint harden in a paper cup and the result was a substance that
was about the consistency of a hard rubber ball. In fact, it bounced just like
a toy rubber ball. Pretty amazing stuff.
The gist of what I've learned is if you want guaranteed trouble in the
future, gel coat your airplane. If you want a finish that is easy to repair,
offers
excellent UV protection, won't ever crack and will look great for many years,
use 2 part urethane paint. The brand we've been using is Sikkens. It ain't
cheap, but neither is refinishing gel coat. From what I've learned from our
resident refinish guru, gelcoat is a thing of the past.
Regards,
John Lawton
Dunlap, TN
A-245
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