On 11/6/2014 7:46 PM, Ed Huntress wrote:
On Fri, 07 Nov 2014 08:35:49 +0700, John B. Slocomb
wrote:

On Thu, 06 Nov 2014 08:28:51 -0500, Ed Huntress
wrote:

John, this has been bugging the heck out of me -- I really hate it
when I don't have a precise answer, so I did some checking to see that
the story is on polyester gelcooat resin.

Quality commercial gelcoats are made from an entirely different resin
than laminating resin. Gelcoats are based on ISO/NPG
(Isophthalic/Neopentyl Glychol) resins, which are much stronger,
clearer, and more scratch-resistant than ordinary laminating resin,
which is orthophthalic resin. In the course of checking this out I
read that high-quality boatbuilders are switching to ISO/NPG resin for
laminating, too, but that wasn't true when I was involved with it, so
I don't know anything about that.

You no doubt know that common plastic laminating resins, particularly
epoxy, have no natural thixotropic properties. Spray or brush them on
a vertical surface, and they drool. Commerical polyester gelcoat
resins are loaded with thixotropic agents so you can lay on a fairly
thick, even coat that doesn't drool. If you cut all the way through
the gelcoat on a cheap boat, you may find that the gelcoat is very
uneven in thickness. Sometimes you can see this as ripples on the
outside surface, because shrinkage upon curing makes the
thick-and-thin stand out. But most builders conquered that problem
decades ago. In the US, they use commercial gelcoat resin from the
start.

FWIW, even ISO/NPG resin, compared to most other resin systems, is
weak and brittle. Without the glass fiber in it for reinforcement, a
good gelcoat resin is mechanically much weaker than the laminate
beneath it. But it does the job acceptably well. And, as you pointed
out, it's cheap.g

Ed, I think I wrote something like "I saw local guys..." or "The way
that they do it here...". I can assure you that people here (however
unlettered) are mixing a color pigment with polyester resin and
slather it in the molds. Just about every "fiberglass shop" carries
the pigments, it is that common.

Oh, you did, and I don't doubt that they do it.


I think that the reason is that "proper" gel coat is very likely more
expensive and probably not as easily available in local shops so the
local guys don't use it as even with the recent doubling of the
minimum wage here materials are still the most expensive part of job
costs :-0

I don't doubt that, either. But if I recall, somewhere in that
discussion was the statement, or the implication, that gelcoat is
nothing but laminating resin with pigment in it.

Not at all.
But you COULD say that gelcoat is nothing but laminating resin with WAX
added...




I just wanted to clear up that point, in case anyone took that away
from what we were saying.

Mostly, I wanted to check my memory and clear it up for myself. I have
some vivid memories of that work, but it was a *long* time ago.