Hi, A customer of mine showed me their sills... ALL SCRATCHED !!! they asked me wether my brush caused it ? I replied...no chance at all because my brushes are specially designed for window cleaners i can show u if u like They replied back to me saying no its okay we were just wondering so and so, basically i think my brush can scratch it... ive noticed that when im scrubbing sideways on the windows my brush head is marking along on the sills so im now being careful. I gather that many of us have had this problem as ive seen a lot of threads about this before.