The stuff I use is called OSPHO (probably another name for Phospo listed above):

Product Details
OSPHO is a balnced formula of phosphoric, dichromate, wetting agents and extenders.Rusted Metal - OSPHO is a primer - NOT A PAINT. You do not have to remove tight rust. Merely remove loose paint and rust scale, dirt, oil, grease and other accumulation with a wire brush - apply a coat of OSPHO as it comes in the container - let dry overnight, then apply the paint. When applied to rusted surfaces, OSPHO causes iron oxide (rust) to chemically change to iron phosphate - an inert, hard substance that turns the metal black. Where rust is exceedingly heavy, two coats of OSPHO may be necessary to thoroughly penetrate and blacken the surface to be painted. A dry, powdery, grayish-white surface usually develops. Brush off any loose powder before painting.One gallon covers 600 square feet. Recommended for use under oil base primer or paint,

Great for use on rusted metal.

Clayton