Some years ago (I don't remember when, and unfortunately can't find it now), a Model Railroader article mentioned using a mixture of PollyS Grimy Black paint and baking soda to paint the wheel faces of friction-bearing (aka "solid bearing") freight car trucks. The idea was to replicate the caked-on mixture of journal oil and dirt that was so common during the Steam Era.
So when it came time to build my first steam-era resin freight cars, I decided to go all out and try this technique (though I used my "grimy black alternative" craft paint instead of PollyS). As you can see (especially if you click on the image to enlarge the view), the effect is pretty great:
|Click here for more photos|