Antique stained glass restoral?

I have a piece of 70 yo antique stained glass that I want to restore myself. someone told me to make a fine paste out of plaster of parris and water and rub thouroughly into where the lead touches the glass and then wipe off with a sponge then polish with terry cloth towel. He says this will not only clean the glass but seal up any leaks as well. does any one know if this really works? thanks