If you read the splurge that comes printed with the aeriel brush it tells you to use a different brush for an initial clean and then swap over to the red one to finish so as not to get the red one "contaminated".
Yet this red brush is sold as a sill brush and in practice wiping sills must be the finest way to contaminate any brush, it certainly is for me and i clean windows(and sills) for a living.
I dont think this brush was designed or tested by a working window cleaner,i`ve cut off the very sharp corners of the stock and fixed some draught excluder cushion to the edges of mine to make it safer to use. Make no mistake the rock hard stock and its rock hard very sharp pointed edges are a liability.It does have a lovely soft glide on the glass but at the end of the day i have to scrub the dirt off the glass not stroke it to orgasm. I get some windows where thats probably all thats required but they would be in the minority.