Gunk Aluminized Cooling System Stop Leak stops leaks in radiators instantly and prevents new leaks from developing. Works in both water and anti-freeze solutions. Draining and flushing before application is not necessary, unless system is extremely corroded or dirty. Contains rust inhibitor. Will not harm metal, rubber hoses, gaskets, or any part of the cooling system when used as directed. For use on car, tractor, or small trucks.