I have never seen anything work like JB Weld. However when I was a snotty nose kid the word at the shadetree garage was, cook up a few gallons of potato soup and empty the radiator and fill it with the soup. Run it through the engine for a few minutes then drain it all out; let the engine cool and refill the radiator. This, they always said would seal any holes in both radiator and engine.
Probably an urban myth...
