Stp. Radiator sealer permanently seals leaks. Stp radiator sealer is designed to repair radiator leaks quickly and effectively, forming a permanent seal. It contains corrosion inhibitors to help prevent the formation of rust and deposits. Stp radiator sealer also lubricates the water pump. Directions. Stp radiator sealer should be used as follows. Run the engine until the coolant is warm (not hot). Shake bottle well and pour entire bottle of stp radiator sealer into radiator. Run the engine for approximately 10 minutes or drive a short distance, allowing seal to harden. Be careful when opening the radiator/cooling system: hot coolant or steam might escape. One bottle treats cooling systems up to 14 litres capacity. Is it safe to use in my vehicle. Stp radiator sealer is suitable for all types of radiator, all types of antifreeze and safe for rubber hoses, metal parts and gaskets. Take note. For best results, especially for older vehicles, clean the system with stp radiator flush, to remove rust, lime scale and deposits before adding stp radiator sealer.