hi
used to come across this regular when i was trad, it only happens on new leaded windows, the lead needs time to weather, used to spend ages getting it of , only for it to re appear after rain.
after a few cleans it went away , so i used to tell custys with new leads that the lead will stain ,but after a few cleans it would stop,
had it again once since ive gone wfp, and i have found a good scrub with ciff works well , but you have to rinse well,
the staining will come back, although not as bad , and will dissapear with further cleans
daz