what we use are fault report forms which we had made in duplicate books and what we do is as soon as we find a fault with the window report it to the site manager (Sm). We had to start using these as if you work for the larger developers (wimpey, bovis barretts etc) they will counter charge you for the damage. The important thing is to have the date AND the time discovered and the nature of the fault (eg scratches). It is important to report it to the sm before you clean the window and get them to sign the form and give them the duplicate for their records and to keep the original safe. It is a bit of a pain but at least it covers you to some extent. As for a disclaimer, from my experience the big guys would tell you to take a run and jump, they don't do disclaimers but you might be able to get smaller companies to sign one i would personally say it's unlikely.