I had only used it ones and was stained by acid rain. Forms of tiny spots (looks like noise) all over the glass. I asked a favor from the canon tech (when I turned in my 70D to them) and confirmed that it was acid rain stains. He was not successful cleaning it and using armor all glass cleaner is not a guarantee due to the possibilities of washing out the coating as well. Any ideas on how to clean this? Lens cleaners didn't work too.