Bob,

IMHE, any metal treatment containing Phosphoric Acid gives steel a sort of dull
gray finish.  I’ve successfully used at least four different Phosphoric Acid
products; all of them did this.

Ospho is actually _intended_ to leave a phosphate coating.

The dull gray coating takes paint well.

It inhibits further rusting, especially if you rub some oil into it.

BTW: Over a year ago, Home Depot stopped carrying Behr Concrete Rust Stain
Remover with the Phosphoric Acid formulation.  This had been my favorite source
of _relatively inexpensive_ Phosphoric Acid.  Worked great on tools!
 
John Ruth
Sent from my iPhone