I will add to this as time permits, a collection of tricks I've learned along the way.
There are a lot of habits and customs that go with any form of antique item restoration. Steam models pose some interesting twists, as they tend to erode their finish by the mere act of firing, and more often than not show up in filthy condition. Due to the need to lubricate externally, they can become quite dirty. And people are not always diligent about using the correct fuel in the burners, so boilers can become caked with soot or residue from incorrect fueling.
My personal philosophy is to get the old engines running whenever possible. That's what they were built to do, to actually operate, not to sit on a shelf and look interesting. If there's one thing more fascinating than an elaborate old Victorian era steam model, it's that same model, actually running. Some burst right back to life, some are cranky and ill mannered after a long period of being dormant, and some just need to be put on a shelf, they're not safe to run.
Believe it or not, these old engines can talk to you in their own way as you clean them up. I recall getting a Bing tractor that had several broken parts, but as I cleaned it up, I couldn't help but notice that the rest of the mechanicals were in excellent shape, everything was coming apart and going back together easily, no signs of wear or damage to the important pieces. This little fellow was telling me something - it wanted to run again. And it did, quite nicely.
There is absolutely no guarantee that a 100 year old steam model is safe to run. These models were built in a time when greater responsibility was expected of the owners, and product liability lawsuits were not nearly as prevalent as they are today. In addition, the passing century may not have been kind to the model. Please reference my safety notes page on some of the things that can go wrong with a century old live steam model. Don't assume that you can just fire up an old model, even if you paid a minor fortune for it.
I tend to polish brass if the boiler were originally turned out in brass. Yes, that may remove the 'age patina', but consider this: age patina on brass is so easily faked that it is almost meaningless. Brass tends to reach it's final shade of aged color within a few years, and there are several ways to accelerate the process.
Many of the Nuremburg models had their boilers protected against tarnish with a oxydised finish, a variation on metal blueing. As the boiler underneath is gold in color, gold+blue = purple, which is why the better engines had a vaguely purple finish on the boiler. I have yet to successfully reproduce this color - gun blueing is too blue, other forms of oxydisation come out brown. Whatever you do, do not polish off that finish if it is present.
Jensen and Empire boilers come nickel plated. Generally speaking, it's a heavy grade nickel plate, but don't take that for granted. If a nickeled boiler is particularly filthy, remove it from the base, clean it first with pure alcohol, then 0000 grade steel wool. This will remove any large pieces of dirt and debris, that otherwise might get embedded in the polishing wheel and scratch the surface. Finally, polish the boiler with a polishing wheel, but don't polish too much - you can go through the plating if you put your mind to it.
All of my polishing is done with jeweler's rouge, it is the least abrasive polish that one can find. Typically, I will use a 6 inch felt wheel on a grinder, but some times have to resort to a dremel with a polishing wheel on the end. The Dremel does not make a good boiler polishing tool, it tends to leave polishing marks behind. That's where the felt wheel comes in, it does not leave marks.
Cleaning the boiler from coats of residue resulting from bad fuel - usually mineral spirits - can be done very carefully with the finest grade steel wool and alcohol. Remove the boiler when doing this - do not spill alcohol on the base. (see next) And rub gently, steel wool will also remove the oxydised finish.
Original paint should be protected at all costs. Reason? Color photography did not exist when most of these models were made. Consequently, the only record we have of the original colors are the remaining examples, so they are still serving as a reference material. Only in cases where there is no original paint at all do I even think about repainting - one of my goals is to preserve actual history, not to preserve my interpretation of history. Rescuing existing paint from age and coats of grime can be done, if approached with modest expectations and extreme caution. DO NOT USE ALCOHOL AS A CLEANING SOLUTION! It will dissolve the original paint, very quickly. It seems odd that a model fueled with alcohol would be painted in alcohol soluble paint, but they are. In some cases, a diluted solution of bleach based household cleaner works well. It will slowly dissolve the paint, but if you work quickly and remove the solution after a few seconds, it will dissolve only a very fine layer of the dirty or aged surface paint, and reveal the true colors underneath. Work carefully, use a white cloth, and examine the cloth frequently for signs of paint, an indication that the solution is dissolving the paint. If you see paint color showing up on the white cloth, rinse the cleaning solution off immediately. For an example of this paint cleaning technique in action, look at the before/after shots of my riveted boiler Jensen 10. Note: Proceed with Caution, Use Common Sense, and Check Frequently For Signs Of Dissolving Paint. You can clean the original paint or litho of these old engines, but you can also destroy the paint very easily, too. When in doubt, just leave it alone.
Freeing stuck boiler fittings.
This can sometimes be done very carefully. More often than not, this is due to deposits being left on exposed threads inside the boiler. Start by moving the stuck piece back and forth. Try to unscrew it, then snug it back down. Give it a firm twist, but don't put too much force, or you may break the nut loose on the inside. If that happens, you will have to disassemble the boiler to fix it, and that is not easy to do. If after nine or ten cycles of loosening it as far as it will go then tightening back down, you don't feel that it's progressing any further down the threads, then back off and call it a day - you'll probably pop the boiler nut loose if you continue. However, if you do feel that it is unscrewing a litte further each time, then start putting very light oil on the threads when you have the fitting as far off as it will go, then screw it back in and back out again. Keep this up, and eventually the fitting will come off - you are slowly scraping the deposits from the thread on the inside of the boiler. Again, exercise caution, and don't scream at me if the boiler nut comes loose - you have been warned.
Vinegar in the boiler:
an old trick to clean out deposits. I did not have a great deal of success with this method, and it stinks to high heavens when you fire an engine with vinegar in the boiler. I suspect decades of firing have created a deposit layer that is difficult to dissolve. But, give it a try. At the least, the vapors will clean out your nostrils.
These can be useful for loosening up stuck boiler nuts, but they will also strip off old nickel plating from fittings very quickly. Be very careful in using an ultrasonic cleaner. Most of the small parts you want to clean are nickel plated.
And the grime that accumulates from repeated lubrication? That's not age patina. It's grime. Clean it off. A good engineer keeps their engines clean and operational, regardless of scale or age.