If you are at 80 grit, you move on when the mirror is to an approximate curve that you want. I generally want the guys in my class to start down through the grits when the depth is about 80% of the final depth. Go on to the 120 or 180 grit and start the process of going down through the grits. Grind until you find the deep pits moving about and then proceed to the next grit. When you get to the fine grits, the Texerau method of glancing a light off of the surface and looking for the even shading of orange as you move the reflection across the surface.
Usually 15 wets will get the glass ready for the next grit but you do have to check to make sure.
I'll also note that going to the fine grits (25 micron and down) should be done only after you are sure that the big pits are all gone.
Also, doing the pencil test is best done after or during a short wet as the grit breaks down and then you don't have an even surface.

• thanks bob - spelger - July 24, 2003 - 20:30 UTC
  • Sphericity - bobmay - July 25, 2003 - 17:30 UTC