PowDuck,
There are several methods that can be used. Several people sale a conditioning stick and or device that has either a diamond stone or regualr stone to surface the face. You can also use a heavy grit emory cloth. You can even etch the glass to help in the condintioning. I use either an emory cloth or a rough sharpening stone but plan on buying one of the conditioning sticks soon. I sand the glass in one direction until it's roughed up. Don't sand too much or the glass will become smooth. Then I turn the call 180 degrees and sand straight across again.

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