[Tosh]

Glass

[jk999]

Oh OK only  stuff I have ever come across that's a ball ache is lime scale and even that comes off with a bit of hard scrubbing with a non scratch scourer or scraper blade, I always bill them for stuff like that tell them from start if they want stuff like that getting off windows pay for the job doing other wise just a normal clean of windows frames and sills if not happy find someone else

[slap bash]

I used Doctor Power and a battery operated drill with a  3 inch round scrauer  pad fitted.

[burky-boy]

I have used this on a new build that had bad stains on the windows it came off with ease but only did a test patch to show the customer http://www.plumbingforless.co.uk/plumbing/polypipe-soil-and-vent-fittings/consumables/polypipe-cleaning-fluid-250ml-cf250

[Susan Dean (1stclean)]

lemon cif with a sill brush ? works great on removeing stains like this on pvc

[Dave Willis]

Tried a squirt of Cillit Bang but that doesn't touch it. Worked pretty well on some hard water splashes on another property though.