When that happens it is a real swine, and it will happen every time if you are not careful
The sunlight has oxidised the black rubber and it wont stop coming off no matter what you do.
Is it an old, early version of UPVC window? it usually is.
The problem will be there everytime you clean the windows.
When it happened to me I just told the customer there was nothing you could do about it, and also demonstrated the problem so she could see for herself.
She was cool about it, and we just had to be more careful with detailing, but we still got the odd mark though.
Ian