I use that silver powder stop leak that you find in any parts stores, many of them have it at the counter/register...
It finds the leaks and plugs them when used properly... Pour about 1/*4 - 1/3 of the tube in the radiator when it's cold, then start the engine and have it run for 30 - 40 minutes for the stuff to work....
My buddy's high school son had a friend that had a monte carlo with about a 1/4" chunk of gasket missing from his water pump... I poured that in his radiator and we ran the engine for 40 minutes... We did that so he could get it home and change the gasket a day or two later... It sealed so well that he sold the car two weeks later and never changed the gasket and it stopped leaking... I could not believe that it worked on that big of a leak....