(NEED TO ADD A PIC HERE)
To do a wash simply thin a paint about 1 part paint to 8 parts thinner. Apply it with a large brush in vertical strokes on the car. This will leave a streaking on the car, very similar to the real thing. If you don't like the streaked look, simply use an eraser at the end of a pencil to erase the thinned paint. I used this technique on the covered hopper below. It leaves a look that is very characteristic of a railroad car.
The white areas on the car depict spills. I will cover this technique in the "streaking" page.