You don''t want to use mat with epoxy. Mat has a binder in it that holds it together. The binder will disolve in polyester resin and allow the random strands in the mat to conform to the surface. Epoxy will not disolve the binder so the mat will not flow out to conform to the surface as well. It also will leave the mat a hazy white color as if it is not saturated. As you can probably guess I already made that mistake and had to call the fiberglass supplier to find out what I did wrong.