Some times the only way is sharp razors and industry known usage methods.
Unless you want to play about with chemicals/acids.
Put you price in (price per hour - you can never be sure until you finish how long it will take, so give a increased estimate)and advise of the issues, such as scratching of the glass, get a damage waiver signed too.
I agree the windows should have been properly covered up before rendering was taken place, but then again, its money for you if you get it, so cant complain
Got one myself today.. i like shoddy builders , plasterers and painters