How to Select the Correct Diamond Concrete 04 03 2021· If you re using a concrete floor grinder on a coarse surface you want a diamond with a coarser grit 30 to 40 should do the trick If you want to polish your surface you ll need a tool with higher grit Anything above a 40 grit will give you a smoother surface Keep in mind that higher grit grinding cup wheels take longer to use