Apply the Ospho and let it dry overnight.
The next day there will be a white dusty residue which can be brushed/blown away, or can be removed with a rag wet with Ospho.
The white residue is a byproduct of the conversion of ferrous oxide to ferrous phosphate.
Using a rag wet with Ospho should not create any more white residue, since there should be no more chemical reaction going on.
After the residue has been removed, I wipe the metal down with acetone, then prime with Rustoleum Rusty metal primer and then final color.
I would not use POR-15 on top of Ospho since it is designed to stick to rusty metal. (I had a total failure from trying that once.) It isn't needed anyway.
Carl
