Have a backglass covered with water ( or chemical ) stains that have not come out so far.  They may very well be etched there for good, but I'm going to keep trying.  Anyone have some good advice for what to use ?

For very hard stain on glass use ''vim'' it will not damage the glass and will remove any hard stain !

Thanks for your suggestion.  I tried VIM and it was not effective on this particular type of stain.  I think the stains are partially etched into the glass, perhaps by something stronger than water.  I was however successful in making the stains less noticeable by using a metal polish, actually CLR metal polish from CDN tire.  Sort of like Novus2, it didn't really start working until the end...when the cloth started "squeaking."  Now the glass shines like crazy and the stains are much less obvious.  If anyone ends up with stubborn stains like this, reach for the metal cleaner !