Ya might try using a lsr [lime scale rust]rust remover. Lime-A-Way is an example and there are others. Muratic acid is a little stonger in intenisity so be sure to read the CAUTION LABLE and be carefull.

Sources: 40 YRS. EXPERENCE

i would try Jewelers Rouge it seems to work pretty well on glass.Also would go to ppg website since they are makers of glass and see if they might have a faq page.Her's another page you might find helpful. http://www.hobbytool.com/jewelers-rouge.htm