My builders went under and didn't finish the job last year and I'm still sorting out the problems a year later. One of the problems is that they fitted the window glass inside out (the lead is on the inside) and I need to get it refitted. Before i can do that I have to clean off some smears and blobs of black stuff on the outside of the windows which won't come off with normal cleaning, I've tried scraping and taking razor blade to it but it's solid. Nervous about scratching the glass. Probably need some kind of solvent but also nervous about damaging tempered glass. I'm sure builders must have something they use to do this post build clean. Can anyone help?