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Coarse diamond hone. They are pretty tough on most sandpaper.

If flatness is out more than a little bit, a line of contact on the metal drum on the end of a belt sander is a good way to go, but...

No, two will just get them to mirror the out of flatness that they have.

I should clarify what I said above about the two stones grading each other and then stopping cutting (i meant stop cutting...

They will grade each other and both nearly stop cutting, but they will release a small number of particles. I think you'll probably find at this point that you have stones with diamonds embedded in...