get the gardiner super lite sill brush and you'll never scratch anything again.
we got some complaints at the start, it was from the brush stock hitting the sill. you can clean without scratching sills, but if you want a fast and easy 100% solution its the sill brush.
i have staff (who will never take as much care as you) and have used this sill brush for 12 motnhs and havent had one complainnt regarding scratches