When prepping a steam locomotive for ops (detailing, painting, decaling, weathering, etc), I like to pre-weather the driving wheels. This is just a thinned coat of Grimy Black sprayed, but applied when the wheels are moving so that I don’t have a masking effect from the side rods.
This is how I do it.
I made up a cradle that holds the locomotive while it is running (below).
The cradle is tilted back with a foam backing. A short piece of track has a pair of feeder wires soldered on and I hold this to the tender with light elastics. The locomotive is sitting on a strip of .125″ x .250″ styrene with blue tack (sticky putty) attaching it to the chassis underside. This keeps the drivers off the cradle for free movement. Each locomotive is a bit different in set up. Engines with pilot and trailing trucks have these removed. Instead of using the strip styrene, I may apply a small wooden block where the pilot and trailing trucks were. These hold the drivers clear.
This assembly is placed into the paint booth and my Digitrax Zephyr is connected. When starting out in DCC, I received some bad advice about using the Zephyr (3 Amp) for an engine terminal. There simply is not enough power in this unit to run lots of locomotives. But since I have one, I now can use it as portable power for utility uses such as wheel cleaning and running drivers for painting. Quite the luxury.
Then I run the drivers at a moderate speed giving the wheels a good blast of paint (I mask the treads with 2mm tape from Jammy Dog [JammyDogTape] beforehand – also using the Zephyr to rotate the wheels while applying).
This is just to get weathering evenly applied initially; later on in the painting process I’ll pay more attention to the rods, links, cylinders and other details. I don’t bother running the drivers at that time.