Andy Dingley wrote:

Fibreglass is generally used with polyester resins, not epoxy.

Not true.

There are a lot of epoxy/glass laminations out there, including my boat.

Epoxy
is used for the really high-end stuff, but it's expensive and a pain
to work with.

Again, I take exception.

Slightly different methods of layup, but it ain't rocket science.

Glass/Epoxy ratios of 50-50 are quite common. Higher with vacuum bagging.

Glass/Polyester ratios of 35-65 are about as good as you can get.


Bottom line....................

Glass/epoxy is much less weight for the same strength.

Simply adding a bit of resin
and maybe mat to plywood will make the surface harder and may make
attachment points less likely to pull out.

Mat is strictly for polyester, not epoxy.

It adds weight, but not strength.

However for real strength
in bending, and especially stiffness and resistance to bending, then
you're more likely to make a double-layered fibreglass beam with two
strong surface skins and a weak filler material between them that's
there as a former, not for strength. You can make surfboards and
airplane wings with just a foam core, so you don't need to be using
heavy plywood any more. You might well find yourself going from "GRP
as an addition" into a full-blown GRP structure design, using a core
that's just the lightest wood (or foam) you've got.

Yep, which is why I asked the question, "What are you building?"

Lew