Think I might try some. Found an outfit called
www.fibreglast.com
in Brookville OH. Want to try it for filleting, especially exterior. 100% silica is always difficult to sand/shape, even the next day. Cotton sounds like you might have a longer window. See how Kevlar responds. Said to be the lightest of the fillers (including silica?) so maybe you can get strength without sag (as I've found anyway, mixing silica and glass powder) using only kevlar.