just as a matter of interest why are you repairing old lead on windows? have you pulled it off with your brush?i have a few customers with dodgy leaded windows.im just extra careful when wfpoling them.ive never once been asked to glue them back on (they always contact a window fitting company, leave them or do it themselves)
Guess it depends if it’s proper old leaded windows, or new mock Tudor types, single pane, that are just fixed over the top.