njjd,

Microballoons will not add strength to the epoxy mix; they simply make it thicker to act as a filler. Milled glass really adds the strength. I've made my own milled fiber mix by shredding some glass cloth and it worked ok. You'll need more than you think you will. When you add the fibers to the resin they really shrink down so the volume that you have when you're creating your pile of milled fibers disappears rather quickly. I couldn't get it as fine as the milled fibers I was later able to buy but like I said it worked ok.