Not everyone is a fan of West Systems but they are very informative.
So I now know that "Stitched mat" is the answer for Epoxy use.
"Can WEST SYSTEM® Epoxy be used to wet out chopped strand mat?
The answer is yes. The fiberglass strand in mat wets out with epoxy, but the binder holding things together does not dissolve. (It does get put into suspension and is sealed in the cured epoxy.)
This undissolved binder causes the wet-out mat to remain a bit stiff compared to wet-out with a styrene-based resin.
For gently curving or flat projects like cabin soles or plywood decks, mat and epoxy should work fine. The fabric does not wet out perfectly clear with epoxy.
Wet-out clarity of mat with epoxy varies somewhat with different suppliers, but none of them wet out as clear as a good 4 oz or 6 oz fiberglass cloth.
Stitched chopped strand mat
There are chopped strand mats produced without a binder that work extremely well with epoxy. These products are referred to as stitched mats.
The randomly oriented strands are held together with light stitching, which runs parallel to the length of the roll and is spaced about 3/16″ to 1/4″ apart over the width of the roll.
The stitching eliminates the need for binders.
These products tend to wet out a bit faster with epoxy and become more flexible on average than generic chopped strand mats.
These stitched mats are suitable for use above and below the waterline for marine applications. Roll mat, the type held together with a binder (not stitched), is used by some of our customers for new construction and repair for below water applications, but we have had some reservations on this if the boat stays in the water."
