I agree with Malc(G)old
Contamination from frames :
two things can happen with the water...
If the water goes over the frame it can pick up dirt or paint....
To see if its picking up paint from the frame, Lick your finger and rub it on the top of the frame,If its white then get a cloth soak it in pure water and keep rubbing the top of the frame until it stops bleeding with paint.. Again do the finger test to see if it has stopped..
If its not that then your water is getting under the seal when you are brushing the top of the window... If so Try this First rinse the whole window first.. Then turn water off then brush as normal .. Then turn water on for the final rinse...
I found that because I was brushing the window and the flow was on, it would spray up into the seal..
Hope that helps... Let us know how you get on
