Triple x wire wool on wet sill will get scratches off easily. But are you 100% sure it's you/your lad doing it? One of my customers was adamant I was doing the same on her brand new windows which I found hard to believe as wasn't happening on anyone else's. I cleaned the scratches off for her and left. Next time I came, to my relief, the scratches were back before I'd even started and luckily she was in so had no hiding place and had to admit it wasn't me. Upon further questioning, she was leaning out of the window with a hand brush sweeping the sills down which was leaving black lines.