: 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.

The only disadvantages would be the cost and the availability of this stuff. Sometimes it is not easy (or cheap) to find these in the quantities you might need. You use it by the fraction of an ounce and buy it by the pound.

You can easily make a good substitute, though. Just use a scissors to cut 1/4 inch strips from the remnants and edges of waste glass cloth. Pull out the long cross fibers and you have a pile of 1/4-inch-long glass fibers in front of you. If that is too messy, pull out four or five cross strands from the fabric first, to reveal a fuzzy edge of glass fibers waiting for you to trim them off with your scissors. When you get enough of these short fibers, add them to your epoxy and mix away. You could make 1/8th inch fibers, too, but you'ld need to make twice as many snips with the scissors.

You can lay those left-over longer fibers along the length of a fillet for added strength if you wish. In effect you have created unidirectional glass fibers -- something you would probably pay a premium to get in any other way.

PGJ