In the past most people thought that the Bar's stop leak in the silver and black bottle was the best.
Recently two different NAPA auto parts store managers told me that K&W Nanotechnology was better. It comes in a bright green plastic bottle. They say it has very fine particles and is better at sealing larger leaks like heater core seams. Some sort of liquid glass that seals permanently. But you have to completely flush out all antifreeze for it to adhere. And it needs to dry for a day.
Anyone have any experience with Nanotechnology cooling system sealer?