Bill Tindall, E.Tn.
The sharpness and ease of cut even from 1 to 3 microns on brass is stark. 5 is almost intolerable. I was surprised

I don't understand what you are saying. What does sharpness and ease mean? What was intolerable? I use diamond on cast iron up to 15 micron , and even occasionally 30 micron, and nothing struck me as intolerable.