In my experience Cabosil doesn't make a more brittle mixture, but it does make a harder final product. Talc, while it works pretty good, makes a softer, easier to sand final product. Maybe that's why the talc mixture didn't chip so easily. The harder cabosil product MAY chip easier...I don't know. It's something to think about and experiment with. Excessive hardener can make epoxy brittle...while too little makes it slightly softer (there are limits to the variance of mixture, of course). If someone else comes up with a theory, or you find the answer with further experimentation, be sure and let us know.