After you have dried the piece, you can coat it with a white paint pen. It is not necessary to cover the piece completely in white pen. The white marks will be taken away by the grinding to give evidence of where you have already ground the piece.
|Piece prepared for the next stage of grinding|
You must make sure the paint has completely dried, or it will wash off with the water used in the grinding.
I find I get best results from this "witness" by making the paint pen marks at right angles to the grinding direction. When all traces of the paint have disappeared, you have thoroughly covered the piece with that grit size. Being ever cautious, I tend to dry, paint and do a second pass at the same grit before going to the next finer grit.