neil
full marks for being concientious, but I think it is a bit unrealistic to try and cover windows during the build process. Windows are built in fully glazed these days, and would require you to try and cover them as they are fitted. It really is the site managers duty to keep every thing as clean and tidy as possible, all you can do is to tackle cleaning 'mucky' glass the right way, that is is use plenty of water, use a good 4''scraper and hope it's not like that next time!
jake