I have been thinking about this problem off and on, and am going to try and describe in words, with a few photos, what my method is, and a few cautions and recommendations during the process.
I begin with tube acrylic paints, available at any art store. There are premium paints and there are inexpensive ones, and for weathering I certainly advise the lower priced brands. I use a lot of Neutral Gray to tone down other colors, along with Ivory Black, and my main brown color, Burnt Umber. For rust colors such as gondola interiors, I would use Raw Sienna (the color of yellow rust) and Burnt Sienna (brick red). I almost never use any white. So that’s about it. Here are my three basic Liquitex colors, and the brush I use for most of this.
At bottom in this photo is one of my cardboard palettes. I usually just cut a square of corrugated cardboard for a palette, for example from a box flap of a cardboard carton.
The weathering process begins by squeezing out a small amount (maybe as much as a small grape) of the black, gray and umber, onto the palette, and then mixing as seems good at the time. I’m mixing in this photo using my brush.
Here I have spread out a piece of newspaper to keep the weathering process off the kitchen table, and have a small dish containing water with just a small drop of dishwashing soap in it, to make “wet water.”
I should mention that models to be weathered must have a flat finish with this method. Water will simply bead up and not spread, on any glossy area, which in turn would mean that the weathering will be pretty unsatisfactory (try it on something discardable as a test).
Next I set up the models to be worked on. For house cars, I usually do the roof only, in one step, then do the sides and ends on another day. That way, you have something dry to hold onto for both steps. Likewise with open-top cars like gondolas and hoppers, I would do the outside in one session, the inside in another. Tank cars are a special challenge, as you really have to do the entire car in one go, and I will say more about that presently.
Once I have a small amount of pigment ready, I clean the brush (a coffee cup of plain water is on hand for this), and wet the model part(s) to be weathered. I just apply the wet water liberally over the area to be done. Here I show a refrigerator car roof being wetted. You can see I am just mopping on the water, not delicately dabbing.
Now that it’s wet, you can add pigment. Before discussing that, I should note that where I live, winters have damper and cooler air, summers warmer and drier. I mention this because it affects the rate at which water evaporates off the model surface you are working on. That in turn sets the time scale on which you can work the wash around on the car.I would advise experimenting to learn what rate you have to work at, on a particular day. In the winter I can work slowly if I like, but in summer I have to work right along.
Next I splodge on some pigment, just dabs here and there on the roof. With experience you will quickly learn about how much to use, for lighter or heavier weathering. This reefer roof is not being given too much pigment so I can achieve a modest amount of dirt. Next, start evening out the pigment with your loaded brush, so that all areas of the roof have about the same amount.
You can see in this photo that the pigment tends to puddle together rather than spread uniformly. Remember, you are not painting, this is a wash. The process now is to stroke the pigment out in each roof panel, making it more uniform, and keeping the strokes in the direction in which water would run off, which on a roof means all strokes are perpendicular to the running board. Repeat until the water has begun to dry, which may be quickly in summer (for me), more gradually in winter.
Once dry areas begin to appear, caution is needed (experiment with not being cautious and you will see what I mean). The shrinking wet areas can still receive brush strokes to even out puddles, but if you stray onto the dry areas, suddenly you are painting, not working with a wash any longer. Results can be interesting, but can also be ruinous. As I say, experiment to learn how this works.
I don’t want this to sound like a small error means it’s all over. Far from it. Acrylics are very forgiving, in that if you don’t like how an area is working out, just scrub it with clean water, the pigment will come off readily, and you can do it over. But do this fairly promptly. Over time, an hour or two, say, the acrylic seems to take much more of a set, and becomes pretty resistant to water.
Roofs are relatively easier than car sides. I will turn to car sides in the next post on this topic.