I'd stay well clear of builders cleans, existing customer or not. If it scratches they'll be blaming you and then you got to check your insurance covers you for the glass you're working on. Not worth it IMO.
If you're determined to do it, I'd get a brand new scraper, plenty of soapy water and scrape in one direction only, washing the blade in the bucket after each pass. Any sand in that scraper, even a grain, is gonna scratch.