The rusting of a galvanised iron tank often is due either to soft water having been used or to the water being softened by heat; the latter would be the case when the tank is above a gas engine exhaust or in a hot position. Galvanised iron cannot resist the action of soft water, and quickly perishes if exposed to such action. A coat of lime-white may delay the rusting, provided it has not got a firm hold. The existence of lime in hard water prevents its having the active effect of soft water on iron, lead, and zinc.