I have been following a discussion on the rec.aviation.homebuilt newsgroup
about gel-coat. I'm not sure if it's relevant to the topic as discussed
here, but maybe someone who knows more than me can set us all straight.
Most of the posting seems to be about gel-coat as I used to understand it -
that is, the thin coat of resin applied to a female mould before any layup
(like in that famous Strand Glass make-a-model-boat-hull demo). Although
the process of application is different (inside-out, in fact), is the
material itself the same as the Schwabbelack (?SP) used for finishing
certain Europas instead of the factory-recommended paint system?
My main concern is that there seems to be a difference of opinon on how
strong (& therefore long-lived) the finish is. Any reliable info available
here?
Anyway, here are some quotes from the thread, edited for brevity:
------------ following quotes from rec.aviation.homebuilt ------------
Mark Fisher wrote:
>
> I am about to make a fuselage shell for a motor glider I am building.
> My question is , if I have intentions of painting the finished product
> with a 2 part paint, is there any advantage in using gelcoat.
> My main concern is weight. Will the use of gelcoat to eliminate
> pin-holes be heavier than aplications of primer to acheive the same
> result.?
Gel coat is used in high production shops to paint the aircraft while in
the molds. You can achieve a decent result by polishing the finished
parts after the airplane is built. But the quality of the finish is much
less durable and attractive as if you were to paint the airplane anyway.
Once you resign yourself to painting the airplane, the results with
primer and paint are as good or better than if it were gel coated and
then painted over that. Gel coat is designed as a primer, but I am not
aware of any advantage of gel coat followed by paint rather than primer
followed by paint. If there is such an advantage, it is limited to
production aircraft like sailplanes. I do not think you will find any
gel coat under the paint of champion quality finishes. The only
advantage I can think of for gel coat is that you can have a smooth,
primed surface out of the molds. The savings is that you will not need
to contour the primer, just scuff it up, fix the pinholes, and paint. If
you are building your aircraft in moldless construction, using gel coat
instead of West systems to fill the weave is expensive and heavy.
Bill Berle
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In article <352448B6.542B@ADDRESS.com>, Bill Berle <FALSE@ADDRESS.com> writes:
>Gel coat is designed as a primer,
If Gelcoat is "designed" as a primer, why is it the finish coat on all
fiberglass
boats under 30 feet. And if gelcoat is less durable why are there thousands
of boats sitting everywhere you look ...unprotected ?
Unmolded construction....there would be better (read lighter) ways. In a mold
there is no other way.
Sal
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Sal,
Take a CLOSE UP look at those boats that have sat out a lot.
The fragile gelcoat probably is shot!
Gelcoat has a very limited life.
Ever notice fiberglass camper shell finishes that look like crap?
More than likely it's gelcoat rather than paint.
Refinishing gelcoat takes many man-hours.
Translate that into $$$$$$$$.
Bob Urban
--------
On Apr 3 1998 12:08:58 GMT, amphibdrv@aol.com (Amphib DRV) wrote:
>If Gelcoat is "designed" as a primer, why is it the finish coat on all
>fiberglass
>boats under 30 feet. And if gelcoat is less durable why are there thousands
>of boats sitting everywhere you look ...unprotected ?
Because it's Cheap, Easy (read: less labor ie. Cheap) and boats aren't
nearly as weight conscious as aeroplanes.
>
>Unmolded construction....there would be better (read lighter) ways. In a mold
>there is no other way.
There are a few. I've seen a straight laquer primer used as a primer in
moulded construction. Mold release-laquer-layups. But I'd want to experiment
with that alot before doing anything large.
Mark Becht
---------
Gelcoat makes a great base but will not eliminate pin holes. I will assume
you are using female molds to make your parts (spraying Gelcoat over a
fiberglass structure is a lot of work and very very heavy). You could spray
a polyester base primer in the mold instead of Gelcoat. The trick is to put
enough on so that it dosen't alligate from the resin softening the primer
without putting it on too heavy. I like working with the Gelcoat
personally. It is much harder than other primers I've worked with, a plus
if your molds are really nice. Finish with some glazing putty and/or
sandable primer, then a good quality epoxy primer and top coat system.
P.S. .
>I am using Derakane vinylester-epoxy resin.
Derakane is a vinylester resin but I didn't know it could have Epoxy blended
with it (oil and water kinda thing).
Louis Kitz
-----------
I often refinish Kestral and other carbon fiber bicycle frames which are
bladder molded (layups of fiber cloth and resin placed inside a mold with
a bladder inside which forces the layups against the mold when the bladder
is inflated).
When out of the mold the surface is rough. To achieve a smooth surface
multiple coats of a heavy primer-surfacer is sprayed on and sanded to
achieve a smooth surface. The problem is that such primer-surfacers
typically contain a lot of talc or other inert fillers that do not have
much mechanical strength on their own. Small impacts, such as stone
bruises, cause the filler to shatter under the paint topcoats. The
finishes chip easily down to the carbon fiber layer.
In contrast, gel coats place a layer of tough resin against the inside of
the mold. This is (hopefully) followed by layups of fiber cloth and resin
applied before full cure of the gelcoat so that a bonding/crosslinking
takes place between the gelcoat and the fiber layups.
The gel coat is tough and typically only takes a bit of sanding to obtain
excellent adhesion with a good primer (two component epoxy works for me)
followed by topcoats (two component urethane works for me).
Of course, letting a gelcoated product sit out in the sun unprotected for
a long time is just asking for the gelcoat to degrade.
Anyway, if Kestral bicycle frames used in mold gelcoats instead of all
that primer-surfacer, it wouldn't take me so long to refinish a frame,
Kestral frame owner wouldn't have frames that chip so easily, and I wouldn't
have so many refinish jobs. Oops.
Hugh Enox
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cheers
Rowland
... that's Rowland with a 'w' ...
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