I recently got access to two different disc repair machines.
The disc repair machines in question is JFJ Easy Pro Plus+ and Disc-Go-Devil from Disc Go Technologies, Inc.
JFJ Easy Pro Plus+:
This machine seems to be good, so far I haven't had any problems with it (see the Note-section below), and all of the discs that I have tried to repair works great and looks a lot better than before.
If you use the coarse sandpaper, you're going to remove any markings that you could feel with your fingernail from the outer edge of the disc into and including the Mastering Code. This might also include the use of soft sandpaper and/or the buffering pad, I'm not sure at the moment.
So, I think you should note/scan/photo that kind of information (Ring code) before you are going to send those kind of discs to someone/company that is going to repair your disc(s).
I'm going to try out the Disc-Go-Devil machine later next week, so I'll provide more information about that disc repair machine when I'm done testing it.
I've got access to approx. 100 different ODDs and a scanner supporting 3 200 x 6 400 DPI.
Blue = Additional information. Red = Change/update information