For a slate call all you want is scotch bright, if it were alum, or crystal you would make it rougher with sandpaper or stone. For alum I use 40 grit on one half and 80 on the other half. Crystal I use 120-150 depending on the sound I want (rougher is raspyier). Knight and Hale SLA-TEk is different, I'm still playing with it to get a desired sound. The goal is to get different raspyness and the different volume is dependent with the surface type.