I need a coarser stone than I currently use to redefine an edge on my chisels. I have an 800 or 1000 grit king stone I believe. The last time I redid the face on my mortise chisel my fingertip was numb for three weeks. I need to place an order with Lee valley and was looking at the sigma power select II 240x grit.

My my question is how do I flatten this? Usually I just use a granite plate and sandpaper. Will this work on the ceramics? Or do I need a diamond plate? Thanks for any help.