I used a Mason jar full of milled 'glass on my last S&G project. I used it to get rid of it. The fibers are very small and fly around easily. It's bad to breathe and has a high itch factor if your not careful. The stuff I had was very old and had wadded up. It took a lot of stirring to make the mix come out smooth.
Wood flour is so much more friendly there's no contest. It's medium strength, cheap and easy to come by and can be made in a lot of different shades.
I have a feeling that putting high strength fillers in S&G fillets is overkill.

Charlie

"Low tech is the best tech"

: In another current thread, srchr/gerald made mention of using milled
: fiberglass to strengthen fillets. I did a Google search and found some
: other references to the use of this material with epoxy - notably also at
: oneoceankayaks.com (used in mounting a footbrace).

: The idea seems to make a lot of sense. How come everyone doesn't routinely
: mix this stuff into fillets? I can't think of any possible disadvantage.