Steve...I would mark the soles with magic marker...diagonal lines from front to back...then put it on some very fine paper, take a couple of swipes and see if the marks wear even or not. Just a couple of swipes, enough to disturb the marks of the marker.
This will tell you if they are "out of flat"...
You could also lay them sole down on your flat granite and see if they rock at all...
For sanding after checking, I start with the finest paper that still marks the sole and then go finer. You will need to judge if you need to go to coarser paper. I like wet paper better. Make sure there are no "specks" on the granite under the paper...
When flattening, make sure you apply even pressure so as not to create a "low" spot in the front or rear. In other words, don't press down on the front when sanding forward and pull down on the rear when sanding backwards...
I'm sure others will chime in with more info...good luck...NICE SCORE, BTW...
This will tell you if they are "out of flat"...
You could also lay them sole down on your flat granite and see if they rock at all...
For sanding after checking, I start with the finest paper that still marks the sole and then go finer. You will need to judge if you need to go to coarser paper. I like wet paper better. Make sure there are no "specks" on the granite under the paper...
When flattening, make sure you apply even pressure so as not to create a "low" spot in the front or rear. In other words, don't press down on the front when sanding forward and pull down on the rear when sanding backwards...
I'm sure others will chime in with more info...good luck...NICE SCORE, BTW...