One application of Ospho stops further rust action. Paint coat adheres so tightly that moisture can't penetrate to the metal. Paint lasts and protects much longer-saving you the time and materials, cost of frequent repainting. Ospho transforms rust to iron phosphate, an inert substance. You will know its doing its job when the metal turns black. This means the chemical change has occurred. Very heavy rust can require two coats. Sometimes, a dry, grayish white powdery film develops. Simply brush off before painting.