It's more likely rust staining.
I've tried various rust removers like CLR before on these, but it didnt seem to help. This last year or two I've been using "Concrete & Metal Prep" from Home Depot to partially derust painted signs, especially white areas. I havent tried it on ad glass, i could see it working but dont know. You could try a spot, let it sit for 5 minutes, and wash it off, see what you get. The metal prep is essentially the same stuff I've been using for years on bare metal before painting in the auto paint business, it removes rust and leaves a phosphate coating on bare metal, on glass I think it would just remove rust, I would think.
Also a chance that evapo-rust could work, but again havent tried that one either.
Maybe someone in here has already tried these before on ad glasses?
I have ruined these before by soaking them for a long time (hours) in solution, whatever you do I wouldnt soak for long periods of time, 5-10 minutes, that kind of thing.
