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kboatright1(at)comcast.ne
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 Posted: Sun Dec 16, 2007 5:28 am Post subject: Gel Coated Fairings |
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Gareys
In general, gelcoat is your friend. It supplies a smooth pinhole free
surface that is more or less ready to paint. Unless your gelcoat is damaged
(spiderweb cracks), or the part needs to be seriously reworked by cutting
away part or adding to the part, there is no need to remove it. In fact,
you'll be creating a lot of work for yourself because you'll have to
completely refinish the part once you sand it down to the glass.
I believe the wingtips and empennage tips are made with polyester resin and
are still supplied with gelcoat. Cowlings and landing gear fairings are
from epoxy resin and don't have gelcoat.
On getting a nice skin/fairing line, I don't know any specific techniques
other than fit, cut, fill, sand, and repeat until you're happy with the
finished product. Sorry!
Kyle Boatright
2001 RV-6
N46KB
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webmaster(at)flion.com
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| Posted: Sun Dec 16, 2007 6:10 am Post subject: Gel Coated Fairings |
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I can't answer all your questions definitively and the ones I can are as
much opinion as fact, so you'll likely get more than one answer from the
list. But here goes:
Gel coat is most commonly used over poly; it's often just a colored
finishing resin painted into the mold before the first ply is laid down with
plying resin. Its purpose is to provide a finished surface. In practical
use, this is almost never the final surface because seams will be visible
or, in the case of our RVs, the molded color will not be the desired finish
color. I've flown competition model sailplanes for year with molded
components and the competitors usually leave them alone; lightness is more
important than cosmetic detail.
Gelcoat can be sanded, use a fine grit so you don't gouge the gel or dig
into the weave. You just want enough "tooth" for your primer; sand until
the shine is gone. Wipe with a solvent or alcohol (I like MEK) to remove
dust and any mold release (gelcoat itself isn't a mold release, so the mold
is usually first coated with release). Flexing should not be a problem for
your finish; you shouldn't have to squeeze the fairing much to get it on.
The biggest problem area will be filling the gap between the parts. Filler
tends to crack and while epoxy can be used over poly, filling with resin
usually leaves a brittle edge.
What you should do is fit the fairing with a little gap built in, say 1/16"
to 1/8". Then go back and carefully sand the sharp molded corners working
along the surface of the piece, not down into it. This will enlarge the gap
but not cut down into the weave. Both the inside and outside corners of the
joggle should be rounded; the main purpose is to ensure that the entire area
has been sanded to receive the fill. The flat area of the joggle should
also be lightly sanded. Mix up some epoxy filler, using flox until it is
fairly stiff. Wax or mold release the empennage along the joint both
inside, outside, and along the edge (if it were me, I'd do this with clecos
rather than after the screws and nutplates are fitted, to avoid epoxy in bad
places). Paint the gap with extra epoxy and apply the filler. Let harden
and then sand the joint smooth before carefully separating the parts. That
should leave you with an exact fit that won't easily crack.
By the way, consider the wingtip VOR antenna; saves you from having to make
the empennage fairing removable and from having the antenna sticking out of
the tail. The instructions for making the joint still apply except that you
can fill after the fairings have been riveted on and you can obviously skip
the mold release.
Pat Kelley - RV-6A - Making antenna cable runs to the panel.
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