It's important to note that Ospho is a rust inhibitor coating and not paint. Before applying, use a wire brush or wire wheel to remove loose paint, rust scales, dirt, oil, and anything else accumulated on the surface. After doing this, apply a coat of Ospho and let it dry (overnight). Once the surface is dry, you can paint over it.
How does it work?
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.
Bare or New Metal:
Make sure new metal is clean and free of oils or grease before applying Ospho. After drying overnight, the surface is ready for paint.
Treat galvanized metals differently, depending on how important appearance is. If appearance is important, apply a singe coat of Ospho and let stand for 30 minutes or until metal is etched. Flush with water or wipe to a smooth finish. Once dry, the surface is ready for paint.
If appearance doesn't matter, disregard the above. Simply apply a single coat, let dry overnight, brush off powdery residue, and paint.
Painting Over Ospho
Painting treated surfaces tends to yield more durable finishes, as moisture and oxygen are effectively sealed away from the metal. Unlike thick paints, Ospho is as thin as water, going on easily and providing much more coverage (about 600 sq ft per gallon).
This metal treatment works best with oil based paints and primers. Be sure to test before using with epoxy or latex paints. Suitable for use indoors or out.
Additional Instructions, Recommendations & Tips:
For best results, do not apply Ospho below 36 degrees F. Also, excessive moisture or humidity may cause longer drying times and increase powdery residue buildup.