I will tell you what does scratch glass, the fookin side of the brush. My upstairs window has 2 bad ones where I have been scrubbing.
Velcro, a small thin strip cut and put on with aroldite stuff. I got the ruff side facing the glass. Been around 3 week's now and it's still working but beginning to wear. New stuff going on tomorrow. Try it and see for yourselves. It definitely does not scratch my glass and I have tested it some.