As Chuck H notes, a case hardened surface is more resistant to corrosion than an unhardened surface, even after the colors are long gone. Also, a highly polished surface is less apt to rust because it has a lot less surface area for moisture and oxygen to react with the steel. While phosphoric acid does leave a chemical coat that resists ferric oxide formation for a time, it will also etch the surface, and is very unattractive for use on guns. It is best left as a cleaner for steel that will be primed and painted.
Unless you plan on neglecting your gun, an occasional coat of good wax is probably all you need.
