This is a case of the rubber seals having perished/oxidized, the only way it will stop is if they are ever replaced.
If they are in UPVC windows, should you rub your scrim from the seals and onto the frames you get a horrible black smear that you can't get rid of
By the time you have finished detailing the window there can be black marks all over the frames, can look awful
I guess you could use something such as T cut to polish it out, but as these seals are almost certainly in pretty old and weathered UPVC frames you would have to do the entire frame Cry Otherwise you will have clean spots all over the place that will look just as bad
I just tell the customer straight, the seals are shot, nothing we can do Cry I do tell them I could T cut the frames, but I would have to charge them several quid per window, and it won't get rid of the problem of the perished seals.
You just have to be real careful on future cleans and detail with a great deal of care.
I am a WFP'er now, so this is no longer a problem, touch wood, I have had no problems as yet with this method of cleaning those type of seals.
Ian