G'day all painters,
Some of the pros and cons of gelcoat do not seem to have been mentioned
yet, so here goes.
There are advantages and disadvantages in using gelcoat.
Surprisingly the positives include brittleness and the "poor adhesion"
with the substrate. This is a most valuable indicator of underlying
damage. We had a rudder flutter incident with a glider which landed
successfully after the incident. The gel coat around the base of the
fin had flaked away making the extensive underlying damage obvious. I
have also seen polyurethane paint elastically stretch over cracks in the
substrate. With the possible exception of gelcoat flaking off after
very rapid descent at some thousands of feet per minute from altitudes
in excess of 10,000 ft gelcoat otherwise adheres to the substrate well.
The negatives include yellowing with age (not very important) and
shrinkage. The shrinkage causes some gel coats to crack along the fine
sanding lines which were produced in the profiling of the surface.
These cracks can propagate into the substrate which requires very
expensive repairs. In my case the cracks had not entered the substrate,
but still cost me almost the full value of the glider to have the gel
coat removed and refinished. However not all gel coats have this
shrinkage problem - you should ask users which ones do not shrink and
what can be done to reduce shrinkage. I am told that Ferro is one which
does not shrink and crack and that the gel coat should not be thinned
for spraying. Spraying also requires a special spray gun.
John Anselmi.
Neville Eyre wrote:
>I spent 29 years in the boat trade, and have more than a little experience with
painting / gelcoat in a far more harsh environment than most aircraft will ever
see.
>A gelcoat finnish will allways be heavier than a properly executed expancell /
eurathane paint finnish, the gel is a very dense material.
>The adhesion of the polyester gel to the epoxy skins is weaker than staying
>with
epoxy all the way through, this can be proven easilly by making test samples
for both systems, and bending them until one of them parts, it will be the gel
that comes off first every time.
>Most gel coats have enough UV barrier to keep the pigments in the gel from
>fading
[ this is also true of cheaper eurathanes ], they do not protect the substrate.
>Back in 1976, I painted a test piece with International Paints Perfection 709,[
the leading yacht eurathane at the time ] and mounted it on a south facing
slopeing
roof. It was still there last year when I visited my old colleages, still
looks as good now as the day I put it up there ! That is 26 years ago ! Gelcoats
will need constant pollishing to keep them shiny.
>The newer family of 2 pack eurathanes are even better in this respect, exept
>for
the water reduced types which do not have the same performance as the good
''old cyanide'' types.
>Precautions must be taken when spraying the cyanide types, a full overall and
an air fed mask, not a big deal, go with the eurathane.
>Methinks someone is touting for work ?
>Happy building / painting !
>Nev.
>
>
>
>
>>>>Gerry Holland <gnh@ddsc.demon.co.uk> 10/19/02 10:14pm >>>
>>>>
>>>>
>"In 30 years all painted Europas will be on their 6th respray
>and will need 120 HP to get airborne."
>
>In the light of Gary McKirdy's prediction as quoted above. How many of
>the pioneering Europas built and painted since 199* have had multiple
>resprays or had engines uprated in preparation of the inevitable weight
>increase due to yet another respray!
>
>For those of us who have opted for Paint not Gel-coat I guess we are
>running a very expensive aircraft. I'm having enough trouble selecting
>my first engine let alone my second to cope with weight increase.
>May be a Europa twin? [musing].
>
>Regards
>
>Gerry Holland
>
>EuropaTrigear 384
>G-FIZY
>
>
>
>
--
John Anselmi,
20 Jones St.,
Stawell,
Victoria 3380,
Australia
Ph. 03 5358 2713
Fax 03 5358 2713
e-mail janselmi@netconnect.com.au
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