These sealants are chemicals that are designed to bond to the internal surfaces in your radiator: They react with trace amounts of oxygen found near leaks and connect with the surrounding metal, essentially patching it from the inside. So yes, these products really do work - the big question is how long they work. Most stop leak products tend to promise “permanent” seals that get your radiator back to new. This is rarely true: A leaky radiator is likely to develop more leaks in the future and should be taken to a mechanic for a closer look, even if the stop leak temporarily fixes your problem. It’s not a good idea to keep adding stop leaks, because they can overwhelm your engine with repeated use.