above you have great advises . i would like to add 1 more . you don't have to spend 100 or 50 or 20 for lapping stone.
check ebay there is guy selling dmt dual side 150/600 it is made in china and price is 9$ .i think shipping include. i use it every time if i need lap any stone. it does leave a little scratches but it isn't a big problem. hope this helps.