Posted By Bob Chester
I have used a sharpening stone to create a bevel BUT I always did so before I started grinding, with a little bit of bevel maintanance done during rough grinding. Now that you have it already polished and a sharp edge I would suggest using a very fine grit oil stone lubricated with water and be very careful of the grit that tends to shed off the stone. The last thing you need is a 360 or 400 grit scratch in your nicely polished surface. Also, the sharp edge may tend to chip a bit as you grind the bevel so be gentle.
Others may have better advice on putting a bevel on an already polished mirror. On your next mirror (mirror making is addictive) bevel the edge first and check it again after you have rough ground the desired curve.
Good luck to you and clear skies,Bob