Sorry I didn't check back on this thread. Dave is right. Most fiberglass mat is manufactured with a polyester binding agent that dissolves when wetted out with resin. The epoxy doesn't break down the binder which makes it want to stay flat instead of conforming to corners. Gougeon brothers (makers of West System) makes a mat that is binder free, but it's difficult to work with because it shreds apart so easy, and it's expensive.