good point - nearly all leaded windows are ancient and should be treated with the care and respect normally afforded to Egyptian mummies or explosives. I had a couple of nasty ones where the water leaked inside, just don't brush too hard but rinse really well, other than that it should be fine - are you using wfp yet?
I do an old hotel with 80% leaded windows; real leaded mostly. The windows are that old Roy Harding did the hotel when he was in his twenties.
There's a '1874' scratched on a window in the 'new wing' of the hotel, so I reckon some of the leaded windows are around 200+ years old.
Some leak, some are fixed with selotape and last month I knocked a diamond-shaped pane of glass out from the inside.
It used to take two of us, two hours to do the outside. It now takes one of us one hour with WFP, and it's £120.00! My best paid job to date.
We end up skiving round the back of the hotel, saying, 'They're gonna sack us', because it's just too quick, so we end up drinking coffee and chewing the fat with the chefs.