The grittiness you were feeling may just have been the scratch pattern left from the sandpaper. On a waterstone I don't think that's an issue. Lots of us lap our Nortons with 325-grit plates and don't think about any scratch pattern left behind.

Some natural hones, especially novaculites (Arkansas, Charnley Forest, and other natural oil hones) do benefit from smoother surfaces. My own Charnley hones much better since I maxed out its smoothness with a flat piece of plain steel.

Best wishes