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Click on the individual
epoxy type for a close-up look.
The plastic 'light shield' covered the middle band between the green dots
at the edges of the photo(and between the names on the blowups). Note that
the covered 'epoxy only' portion is lighter than the exposed epoxy above
but the opposite seems to be true for the varnished band. The darker hued
band under the black line is the varnished epoxy that was light-deprived.
The exposed 'varnished epoxy', which is of most interest did not change
that radically in any sample which was expected after such a short period.
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 Observations
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Fabric 'whitening'
- Fibers that either didn't completely wet-out (although it looked like
they did initially) or can be seen through the epoxy (the fabric weave
is close to the surface) show signs of whitening. This makes a few strands
of the fiberglass clearly visible inside the epoxy matrix. Click here
to see the effect (inside the circle).
On a close examination, this happened ONLY where the epoxy coating is
very thin or the weave of the cloth prints through. This phenomenon is
completely absent on thickly coated patches. Since the fibers were clearly
wet out when the panel was new, it is a bit of a mystery why this occurred.
Water penetration could be one possible answer. Early UV damage is second
on my mind but epoxy shrinkage seems to be the most probable cause.
The shrinking epoxy might have delaminated (on a microscopic level) from
the fibers.
The lesson seems to be: Make certain to bury the cloth in sufficient filler
coats of epoxy and especially on sanded-through fiberglass before applying
varnish (yes, 'whitening' is also visible under the varnished patches).
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Epoxy shrinkage
- Even though manufacturers claim very small shrinkage rates on epoxies
(~1-2%), epoxy does shrink and based on the amount of fabric print through,
it seems to shrink far more than 1%. Again, the only way to minimize
this is to apply thicker filler coats. Sanding the varnish off after a
few years and applying a new filler coat of over the stabilized epoxy
might even put an end to fabric print through once and for all. (Just
my theory at this point). I am told by people in the aviation industry
that, based on their experience, some epoxies (on wing skins and fuselages)
might keep shrinking even after 10 years! Who knows, I can't testify to
the validity of this claim.
By the way, fabric print-through cannot be practically covered over by
varnish unless you want to put on 15 coats. After you varnish the kayak
and a few days afterwards, it may look like the varnish has some scratch
filling properties but wait a few weeks and add some exposure to the sun
and the fabric weave will slowly and unmistakably start printing through.
As a note of interest, you can also notice fabric print-through on some
commercial composite kayaks. The reasons for print through in these cases
are: Thin gel coat to save weight, coarse fabric applied right against
the gel coat and also over-thinned polyester or vinylester resins which
shrink even more than epoxies. Click here
to see fabric print through. In this instance it is the West System 206
at the top left of the photo.
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Varnish 'water resistance' - Here again, we are cautioned by manufacturers
that varnish shouldn't be continuously immersed or exposed to water. I
have realized this but ignored this warning, thinking that rain would
just evaporate from my test panel and everything would be fine.
I tested this 'water resistance of varnish' inadvertently by covering
the panel with waterproof plastic. It kept the water out but it also kept
whatever H2O got in! After peeling the strip off I was really surprised
to see the varnish blistered on the 'blushing' epoxies (WS206 and less
so on East System). Despite wet sanding the epoxies prior to varnishing,
some post curing 'blush' must have precipitated UNDER the varnish skin.
The small patches are varnish blisters that peeled off.
This blistering was not evident on the other epoxies but the varnish coat
was very soft (gelatin like) and easily scraped off. After a few days
of drying it turned progressively harder until it became almost as hard
as the exposed varnish. It is clear from this that cured varnish can
absorb water when exposed for long enough period.
See the blistering
on the West System 206 sample.
The moral of the story is to never keep your varnished boat covered for
a long time with non-breathing water proof cover that traps moisture next
to the skin.
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Epoxy changes (unvarnished) - The West System 206 unvarnished surface
turned opaque, showing early signs of UV damage. The East System turned
milky also but to a lesser degree. This opacity is no longer due to 'blushing'
since it cannot be removed.
With the exception of changing hue and saturation of color, the rest of
the epoxies are as clear as new. The System Three and Raka have stayed
remarkably clear and free from fabric whitening. MAS shows few white strands
and West System 207 exhibits a few more ( I think it is due to the epoxy
thickness or partial wetout due to the density of the fabric weave and
higher viscosity of the resin).
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Back to Epoxy
Results page or 6 month panel results
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