If MDR's not working, you could try Jeweller's Rouge, which is a powder. You dip in a moistened cloth and it becomes a paste on the glass, you then use lots of elbow grease! You can get this from some of the larger glazier firms. It works on the same principle as T-Cut for car bodywork, which is probably easier to find. Try some Rouge, or even T-Cut, on a small patch first to see if it works, and then figure out how long the whole thing will take you!