MDR won't shift this bloody stuff, it isn't a mineral deposit, and a scraper is no good either.
At the cost of chemical removal, the customer might as well replace the glazing, £40 per metre will cover the cost of most double glazing panes of glass comfortably.
Because this stuff is transparent, when you are applying it you do not notice it splashing all over the glass, a good decorator will of course, but if it's done by a labourer (with the idea that it doesn't need any skill to apply) then you are left with a real mess
As Roger (Squeeky Clean) will tell you, best to leave well alone!!
Angle grinding near a window ruins the glass too!!
Those little sparks hit the glass and fuse into the surface, after time you end out with rust spots, and absolutely nothing can be done, to rectify the glazing has to be replaced
Regards,
Ian