Use a fine grit sandpaper when sanding Slate, Copper or Aluminum, never use any coarseness lower than 220-grit, finer grit is always better. Scuff pads such as scotch-brite are the best materials to condition surface with. Sand slate, copper and aluminum with a circular pattern and sand lightly.
Use a coarse grit sandpaper when conditioning glass, or crystal. 60 grit sandpaper works well for these materials. When sanding glass or crystal, use a cross-checkered pattern. Sand one direction of the surface and rotate the friction surface 90 degrees and repeat the sanding. Try to get the surface to resemble a cross hatch pattern. This will ensure uniform call conditioning that will work at any angle the striker is applied.
***Always condition the friction surfaces in the same pattern to ensure consistent call quality.