[JandS]

What is it exactly that causes the black streak marks on the edge of the window sill?
Is it something to do with the seals?
Doing it on one of my customers downstairs windows  and the window is only about 5 years old.
Need to explain it to the cantankerous old bar steward.

[zesty]

Must be the black rubber seals, as they perish they start to ‘bleed’

[NWH]

If I pulled up to a job and it had these old seals I’d just walk away they are more hassle than they are worth,you or your water don’t leave spots and runs the blacks seals do these are few and far between these days unless they are dirt cheap windows they all come with hidden seals within the beading these days thank god.
WFP came out in the main in the late 90s these windows were everywhere this in itself gave WFP it’s bad name early on.

[windowswashed]

Flocked brushes entice the perished seals to bleach more than mono filament brushes.
I use rinse brushes for almost everything and this overcomes that problem regardless of flocked or mono filament brush as the clear water from the rinse is fine but the bristles agitate the bleeding effect.

[NWH]

You can talk all day the best way to go about trying to clean these types of windows this brush that brush agitates it etc,honestly the best thing to do is leave it to the bloke with a mop and blade the time he’s finished cleaning windows like that his scrim or Microfiber will look like a Zebra 🦓.

[Smudger]

These windows are easily cured - spray with TFR and scrub - you will get masses of black running out of the seals for around 3 or 4 minutes after that the water runs clear and from then on you don't get any black bleeding in future cleans

Darran

[Splash & dash]

These windows are easily cured - spray with TFR and scrub - you will get masses of black running out of the seals for around 3 or 4 minutes after that the water runs clear and from then on you don't get any black bleeding in future cleans

Darran

This is interesting I have tried everything and once the seals are degraded by UV I have never been able to stop the black  coming off  the seals I just tell customers that the seals are knackered and there is nothing I  can do to stop it.  I have done a restoration on one window as an experiment the frame was restored to almost like new but within a few months the black staining was back again , so now don’t bother .

[Bungle]

These windows are easily cured - spray with TFR and scrub - you will get masses of black running out of the seals for around 3 or 4 minutes after that the water runs clear and from then on you don't get any black bleeding in future cleans

Darran

This is interesting I have tried everything and once the seals are degraded by UV I have never been able to stop the black  coming off  the seals I just tell customers that the seals are knackered and there is nothing I  can do to stop it.  I have done a restoration on one window as an experiment the frame was restored to almost like new but within a few months the black staining was back again , so now don’t bother .

Agree. Once they bleed they don't stop bleeding.

[Dry Clean]

Removing the ingrained marks on the sills will be more trouble than its worth but the windows themselves aren't that hard to clean, you just need to make sure that any water above the seal has stopped dripping before cleaning the glass below it, the same as you would with a problem vent,  there are a number of ways to achieve this.