You'll do more damage with your washer and scrims than you will with the brush, the bristles are way to soft.
Quite apart from which, the varnish or paint would have to be in a dreadfully flaky condition to begin with for it to be lifted off.
Roger is wrong on this one, as almost anyone who has been using WFP for any length of time will tell you
In over 18 months I have lost only 2 or 3 accounts as a result of WFP; Admitedly I didn't have many domestic accounts, only a handful, all my work is Commercial (or was).
Trad methods will cause more damage to frames than WFP, not to UPVC frames, but those frames that are in fact varnished or done with a stain such as Sadolin, these are usually frames where the glass is held in with beading and not putty, so the edges are more or less 'square on' to the glass, this means the edges of the squeegee channel will often come into contact with the beading, and this can mark up and damage the frames.....
But to be honest, who gives a monkeys
In 20 years of using trad I've never had a customer so much as mention it...but I have noticed it mind!
And please don't tell me you always have the perfectly cut length squeegee rubber, it still occurs!!
have fun,
Ian