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.