Peter... search for micks (t20) thread called 'it's a bit rusty'.
It's not the paint on solution you are referring to, instead it's the fume bluing which I imagine takes about the same amount if time. I didn't try the solution bluing.
The fume bluing you can use just about any acid that will cause rust!

Kenny - I think muriatic acid is the US name for hydrochloric acid.
You can still buy hydrochloric acid on fleabay. That's what I used for rust bluing