Phosphoric acid converts rust (orange oxide) to black oxide that creates a barrier to further corrosion. That’s what the POR products use to achieve their results.
Just spray with phosphoric acid in the nooks and crannies and carry on.
That will work, but phosphoric acid makes iron phosphate, not black oxide. Black oxide is magnetite and is formed with a salt, not acid. Black oxide passivates the surface by oxidizing the surface and tying up the free iron.
Phosphate is a conversion coating which creates a physical barrier which is not an oxide, but instead a crystalline layer. It makes a better primer and sealer than oxides do.
