No way around it I'm afraid, that's their problem, they need new windows (lol.)
You can minimize it, but not prevent it.
If you have a digital varistream and a similar set-up as mine (see sig), put the shutoff pressure on 7, flow rate on 1.
As for brushing technique, brush only in downward strokes. From side to side seems to go well too.
A trigger on the pole is even better because you can give short squirts of water during brushing, whilst having it on whilst rinsing.