I use the 12" GP sanding bars too, but I rarely get too critical with it. I have a rough paper and a fine paper, I couldn't tell you what the numbers are but to me, it's really not that important.

What IS important is getting all of the dust off before you cover! I usually pat it down to knock off the loose stuff and then wipe it good with a tack cloth or a towel sprayed with a spray adhesive. You want the covering to stick to the WOOD, not the DUST!