I played around a bit today with some metal hardware. I prepped just as though I was gonna phosphate bath them but instead immersed them in plain ole white vinegar. I wire wheeled them to shiny metal. Left them in about an half hour. They turned medium to dark grey quickly. I immersed them for another treatment. Did not get any darker.
I am sure it's been tried by others on here, so has anyone got longevity reports using this method? I also immersed them into motor oil for a bit.
Longevity? As always depends on your local conditions (humidity) the cars use and exposure. Also how often your re-oil pieces. I've had some parts on one of my cars that started showing a little rust in a couple of areas after letting the car sit for 5 to 8 years in storage. A little steel wool, a touch of the gun bluing repair fluid and a protective coat of oil and they were back to looking nice again. Or you can remove the piece and just redo it - depends on the part. Larger items with that you want completely dark should likely be redone completely to produce a consistent look across the whole surface.
I heat of the parts (not enough to produce any issues with the coloring or metal) before I quench them in oil. May help the metal release any water from the process, open pores in the metal and allow for better retention of the oil over time.
As for the comment about it not getting darker. Guess your dipping or applying directly (one of the two styles that produce different looks) with brush or rag. The colorization takes place almost instantaneously and will not change with multiple applications
