ok i have done this but be very very carefull

use bam the bathroom cleaner

but do not spray it anywhere near you paint, spray it on a scourer sponge have a bucket or 2 of fresh water handy for a quick rinse, take your scourer and scrub the glass with the bam on it, then quickly rinse it off with the fresh water and finally give it a good hosing to clear any residue away.

It is absolutely you get it all off asap as it is acidic and will root your paint if you are not carefull, it's not a strong acid but it'd be enough to cause problems. but it will get any mineralised staining off glass 100% even bore water stains!

good luck