I use the 45min mud, little plaster and cabosil or qcells as a filler,
http://www.shopmaninc.com/fillers.html I also went to the local redimix concrete place and bought a bag of fiber, very fine fiberglass fibers and a bag will last forever, also very cheap (i think I paid 7-8$ 2-3 years ago) just a lil pinch added will give lots of strength.