Fernox F4 Express Central Heating Leak Sealer 400ml Fernox Leak Sealer F4 is suitable to seal small leaks and weeps which may cause pressure loss and boiler breakdown. Quick and easy to dose using adapters supplied with no mess. Features and Benefits: F4 Express Central Heating Leak Sealer 400ml Stops Small Leaks Quick and Convenient – Doses In 30 Seconds Add Via A Radiator, Fernox TF1 Filter or Filling Loop with Adaptors Supplied Treats 130 Litres or Up To 16 Radiators Seeks Out and Seals Most Weeps and Leaks