Stop leak chemicals are only a temporary fix when they work. You will end up replacing the radiator anyway later. The chemicals react to contact with air, and there's air in the reservoir. It will take multiple warm-up and cool-down cycles to draw some of the chemical into the radiator and I doubt it will be enough to be effective. My suggestion is don't waste money on "mechanics in a can". If the radiator is in otherwise good shape, many leaks can be soldered. If the cooling fins are rotten and corroded, why put off the inevitable? Replace it.