mike worrall wrote:

In female molds (on the other hand), gelcoat is the first 'foreign
substance' applied to the mold (release agent assumed). This is
almost always followed by a layer of mat, to avoid the visual
'print-through' of the heavier woven materials in the laminate. Mat
must be applied using polyester / vinylester resin (which contain
styrene, which breaks down the binders in the mat).

Epoxy compatible mat is available. I've used it in a few repairs.