and i guess neither of us mentioned if isopropyl is used as a first step, and it doesn't do the job then an ammonia based cleaner was resorted to in either/or both cases the glass must be cleaned in a follow up with distilled water. 

never let any solution dry on the surface of the glass. 

one may want to reference this excellent article on the sensitive matter of cleaning mirror/glass surfaces 
by keith quattrocchi, md. phd.                                                                                        
                                                                                              
 A Practical Guide to In-Situ Cleaning of an RCOS 16" Mirror

 http://www.ourastrogallery.com/mirror.htm




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