I know some of you have read me writing about this before...though I don't like removing a lot of material from the back, you have to remove some sometimes on an initial tuning. I have used a belt sander to rough it out. But it still has to be flattened after that, and most all of the stones are not coarse enough in my opinion to start. The belt sander is generally pretty coarse. I like to tune on plate glass with psa sandpaper, starting with 80 grit, 150, and 220. Then you can switch to the stones after that or work with finer grits on the glass. A piece of 3/8 or 1/2 inch plate glass is pretty cheap and so is the MDF Frame I make to hold it. My plate glass is about 30 inches long, by 5 or 6 inches wide. It works.