The trouble with leaded windows, if its the traditional style leaded glass then dirt builds up in the channels over the years and takes a lot of brushing and rinsing out, also can (does leak )inside, Iexplain this to the customer and so far they except that problems can happen,
With modern leaded windows (the lead is stuck onto the glass) againd dirt and soap residue can build up along the edges if they have not been boned down correctly, also there are diffrent quality leads and some can start to oxidise a little when first treated with Pure water, again the problem tends to go away after first clean.
I find it a nice little earner replacing damaged lead to modern windows as normal installers can't be botherd to come and replace a few inches of lead,
Alan