One of the things that stick out like a sore thumb on nice models are the shine from the wheels, trucks and couplers. Perhaps you don’t own an airbrush, or don’t want the hassle of the airbrush set-up and clean-up for a small job like this.
I’ve found that acrylic paint is a cheap, quick and odor-free solution to this problem. I use Folk Art Nutmeg as my color of choice for rusty wheels.
First, clean up any flash from the wheel sets and trucks. Then take a #1 brush (lightly loaded with paint) and touch it to the wheel side while gently spinning the axle between your thumb and forefinger.
After completing the wheels, work most of the paint off the brush onto a piece of cardboard (the kit box will do), and brush over the trucks and couplers to highlight the detail. (I’ve found that lightly brushing the copper Kadee knuckle springs improves realism without sacrificing performance.)
As with all weathering, start light; you can always add more paint in subsequent applications. An added advantage of acrylics, over other paints, is that mistakes can be washed away with water if you catch them before they dry. (If you are too late in discovering a mistake, that’s OK — now the entire car is simply a candidate for a full car weathering job!)
I’ve found that the whole process adds less than five minutes to the construction of a car, unless I have to find the brush, which if you’ve been in my basement when the layout is not on tour, you know could add days to the process. I know, I know, this time we’re gonna clean the basement and keep it clean. Yeah Right. Anyway, rust in peace.