In the seal business the term is used to describe seal face flatness, not polish.
Seal faces should be lapped flat to within three helium light bands. This is 0.000033 inches or just under one micron.
Testing has shown that seals begin to leak at about five light bands. The smallest thing that can be seen with the human eye is 40 microns, so we are talking about a very small distance.
Carbon-graphite seal faces should be lapped flat on ceramic stones of varying roughness. If you use lapping powder the abrasive particles will embed into the carbon-graphite mixture and in operation, damage the mating hard face.
Hard seal faces are lapped on ceramic or cast iron plates that have been impregnated with diamond particles.
See: Flatness Readings, GR006
- On February 16, 2018