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A flat glass surface on its own will not have any significant abrasive properties. As such, it will not sharpen steel. My suspicion is that the bare glass can act in a similar manner as a strop; it can straighten out the bevel but won’t remove much (if any) metal. This would restore the cutting ability of a rolled edge that is still sharp.
The glass platens are intended to be used as a hard, flat backing for the polishing films. These films have very fine and uniform abrasives attached to them and can be used to put a clean, flat, and highly refined edge on a blade. I have some of the diamond lapping films and I highly recommend them. I can’t say that I’ve seen anyone say they didn’t like them.
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