..This is the same reply I give to most questions like yours....
Try and find someone in your area who has a hot water pressure washer....sub-conract the job to him..you carry on cleaning windows and you`ll all win.
If that doesn`t help then this might......My company does loads of these and we mix traffic film remover with car wash and wax, put the lot into the karcher, lower the pressure and wash away with the lance. I might add that I have extending lances ( 5.4 mtr ) and they are great. Any stubborn marks are removed with telescopic brushes then all you have to do is ensure that the window ledges are cleaned, usually with a cloth and elbow grease.....not much though
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As I said we do loads of it and once you have the right gear then a semi with cladding and upvc windows can be done in around an hour and you can pull in £80 - £120 easy......There is lots of this work about.
Be careful though......low pressure only. Don`t forget the liability insurance too. Don`t be scared of the prices, the client has asked you `cos they can`t or won`t, they are paying for you, your machinery and your time.
Have Fun...