[Jack Innes]

You are talking about "Metal Prep" which has been the standard final cleaner prior to priming used by body shops for years. There are many different brands & suppliers listed under http://www.google.ca/#hl=en&sugexp=l...w=1075&bih=588
This is phosphoric acid based & works well but leaves a vulnerable surface that needs to be painted or oiled fairly quickly.

Combined with tannic acid in the process I described earlier it will give a stable, black finish that, with a little oil on top, will look good without defeating the patina that is now in place.