A scratch will be a single line of big pits in the glass while a sleek will more look like a wide mark of surface color change. There may also be some big pits, usually at the start of the sleek in a jumble.
Sleeks are more from dry working of the glass and tool while a real scratch will be from a piece of something big between.
That you are getting them on the outer part of the glass, kind of tends to indicate to me that they are sleeks which are more from running the wet dry than anything else. If you are using a tile tool, make sure that the tool is good and wet (assuming that the tool isn't sealed) as a dry tool will adsorb the water you need between the tool and the mirror.
It also is very easy to over grind on the fine grits so you want to more work according to time rather than the sound of the grit as it works. ON that 8" mirror, the stroke should be on the total length of 2" or less with no more than 1" overhang on each side as you are trying to get the mirror spherical more than anything else.
If the mirror persists in getting scratches at the 5micron level, you will need to put the grit into a bottle and use it as a slurry rather than as a dry poweder. This will let the heavy stuff settle out before the grit is used.
I'll also note that you can stop at the 12 micron level although it will take a bit longer to polish the mirror. In addition, you can use the 12 or 5 micron grit as the start for polishing with the lap - yep, it will polish!

• 12 or 5 micron grit as polish... - czar7 - December 8, 2003 - 20:24 UTC
  • Re:12 or 5 micron grit as polish... - optics - December 9, 2003 - 04:20 UTC
    • Thanks! - czar7 - December 9, 2003 - 14:13 UTC
    • Grit polishing. - bobmay - December 9, 2003 - 19:32 UTC
      • Polishing Pads... - czar7 - December 14, 2003 - 22:49 UTC
        • Re:Polishing Pads... - optics - December 16, 2003 - 23:26 UTC
          • Polishing pads.... - jkoptic - December 17, 2003 - 04:49 UTC