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 Huntress