+1 on the Bar Keepers Friend for cleaning lapping plates. I use mainly waterstones in my work and find the finer grit diamond plates maintain flatness quiet well. I will often use a Atoma 400 and then final dress with a finer used paper. I require certain stone finishes with different stones and a cleaned and used piece of 600 paper gives a nice surface finish for use with clean water. Honing with slurry the 400 is perfectly adequate for most synth stones.