It all depends on which side of the mirror you want to sandcarve. If you sandcarve on the front of the mirror, then scratching is the main thing to watch out for. If you plan to sandcarve through the back of the mirror, then you want to make sure not to use any glass cleaning products that contain Ammonium, especially after you completed sandcarving. The Ammonium can cause a chemical reaction with the silvering of the mirror (now slightly exposed where you sandcarved through the backing), which causes black spots to “grow” from the exposed edges.
—Ruth Dobbins, Professional Glass Consultant