From DIYinfo.org
Doors and windows may have leadlight panels, consisting of small panes of glass held together with lead strips that are H-shaped in cross section.
[edit] Dealing With Leaks
Using clear plastic finish
1 Mark the leaks with a wax crayon so that repairs can be undertaken in fine weather.
2 Carefully scrape any dirt away from the edges of the lead strips.
3 With a small artist's brush, paint clear, exterior grade plastic finish liberally along the flanges of the strips. Take great care to seal the outside of the glass.
ALTERNATIVELY, inject a bead of the clear sealant used to seal leaks around car windscreens. 4 Press the flanges down, supporting them firmly from the other side. Wipe varnish off the lead and the glass with a cloth moistened with turps.
[edit] Using putty
If the varnish does not work, the putty in which the glass is set must be replaced. Use soft glazier's metal casement putty. You can colour the putty grey with a little black powder paint.
1 Open up the lead strips slightly on the outside of the window by levering up the edges with a chisel. To do this it may be necessary to cut through the corner joints with a trimming knife or some side-cutting nippers.
2 Scrape out as much of the old putty as possible and clean out the dirt.
3 Press in the new putty.
4 Press the lead strip back in place, with a helper supporting the glass from the inside.
5 If it was necessary to cut the corner joints, glue them back together with two-part acrylic glue.
6 Wipe the excess putty off the lead and the glass.
[edit] Repairing cracked glass
It may not be necessary to fit new glass if the crack is only a minor one. You can try running a few drops of glass bonding adhesive into the crack.
Replacing broken panes in a leadlighting panel is a difficult job. It would be best if you called in a glazier to do it.
[edit] Curing bulges and buckles
Take the complete panel out of the door. It may be held with putty, like an ordinary sheet of glass, or it may be held with both putty and an outer wood beading. You can remove the beading by unscrewing it or, if it is nailed, by prising it out very gently.
Lay the panel on a flat surface and press the lead strips flat. Be very careful not to press too hard or you are likely to break the glass panes.
If this does not work, it may be necessary to take the panel to a glazier to have it rebuilt. While this is being done, cover the opening in the door or window