Did you try what I posted above?
Her a can of Bar Keepers Friend powder.
It is safe and cleans off deposits and will fix that if it is infact something built up on it and not scratched or etched in to the glass.
In just about any store in the cleaning supply section.
Even Wal-Mart has it. Like 3 bucks,a can.

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VP of the MPM in New Orleans
73 Dart Sport 340/ 70 challenger vert. That may still get built, If I live long enough