How to cement (waterproof) a stained glass window
Cementing is the application of an oily putty like mixture to the lead came strips that join the glass in a leaded window. After soldering the panel, cement is brushed between the lead and the glass to secure it, make the panel watertight and as it dries, make it stronger and more rigid.
We recommend applying the cement with a cheap dish brush, you can scoop the cement from the tub and brush it into the lead. Seal one side completely then dust liberally with whiting (this is chalk powder) which draws the oil from the cement and aids drying. Do this on both sides and then brush any cement off the lead (it doesn't need to come off the glass, but do not leave it on the came or solder as it won't come off!)
Leave overnight to dry, then clean using a pointed stick and a scrubbing brush untill the glass is clear. You can now polish your stained glass window to darken the lead and give it a real shine. Use black graphite polish for this. Unfortunately Zebo (Zeebrite) is no longer available due to is lead content, but this is a great replacement.
Leave the panel for a week or so in a warm place before fitting. Or longer if you can wait! If it is installed too soon, the weight of the panel will push the cement out where it is still wet.
You can loosen the cement if it has started to dry by adding a dash of linseed oil and / or white spirit. Only buy what you need to last 3 months or so as it does eventually go off.