This is a popular topic and I'll add my part, having seen many samples of Russia iron from different sources and dating to an 1857-1900 time period.
I strongly recommend Testor's "Gunmetal" Metallizer, a buffable metallizer that is then sealed with a clearcoat. Testor's makes a sealer but I've used many different types with sucess. It has an excellent color and creates a beautiful and accurate effect.
For the engine you are modeling, a dark silver-grey charcoal color is best, and certainaly the most typical for your era. It should not have any overt blue, and I cannot say this enough. Many otherwise excellent models have been badly compromised by "blue moments."
The Testor's method has a good advantage as well, in that it requires the least number of steps (3), and less chance of damage in painting. Your model is very finely detailed, and I think this would lessen the chance of damage in the process.
When you have the boiler sealed, consider a light turpentine wash using oil paints in burnt umber and black, in a 2:1 mix. This will highlight the Russia iron bands and jacket courses, and suggest the use of oil in wiping the jackets during use. It will also give a slight brownish tone similar to one of the suggestions you received, and I think would make your model more accurate and suggestive of an in service engine.
I was backdating an On30 Bachmann T boiler shay to its 1884 appearance. I dissassembled the upperworks, and stripped the boiler section of paint. I was going to use the Testor's Metallizer method, but the white metal boiler was like a group of sirens on the shore, and I had no mast to tie myself to.
I polished the jacket using flexifiles, medium and extra fine. When the jacket was highly polished, I quicky dipped the boiler into an open jar of "Blacken It" for half a second, then immediately held it under running water and polished the jacket with a stiff nylon bristle brush while the water was running over it. The trick is to not let the blackener saturate the inner metal, and to keep it light. I repeated this process maybe three times until I got a finish I liked, then dried it with a towel.
I masked off the boiler and painted the sandbox, and so far I have not seen any problem with paint on the blackened metal. The boiler also had four holes for the generator mount - and as no engine I will ever run will have a generator, I cut the mount off at the surface, using the plastic lugs to fill the holes. They are faintly visible - if you look for them.
I also used the blackener to get a good effect on the firebox and smokebox, by dipping the unpolished metal into the blacker for a longer period of time, maybe five seconds. It gives a great effect and I'm very happy with it. I wasnt planning to do this originaly and the smokebox came out slightly different in tone, but I used thinned Floquil washes to adjust it. On a second engine I would plan this part out better and get a more even match between the two.
The blackened metal / Russia iron is really good, and so far it requires no overcoat, and has not discolored. Its better than any paint, but can only be used when you have a metal surface to work with (Testor's Metallizer is the best for plastic). A lot of the upcoming engines such as the On30 Forney will have metal boilers, so there is a lot of fun to be had.
-Jim Wilke
Model Building / Simulating Russia Iron / Suggestions on simulating Russia Iron