Does anybody know how to paint (passivate) iron (steel) part to black colour ? Not using any enamel only chemically. As I remember I could be done with some mixture of phospohoric acid and .. or chromic acid.Thanks
A process used for rifle parts commonly referred to as 'blacking' causes a controlled corrosion reaction on the steel surface (not oxidization). This forms a protective layer preventing further oxidization. If you like I can look up the exact details of chemicals and quantities for this process.
I have re-finished a gun using a commercial gun "bluing" solution probably obtainable at any gun store. This put a nice shade on the steel and you could make it darker by applying more coats. This is not a paint but chemically changes the surface.