I used some super fine polishing paper to remove some of the lettering from the car.
The rust is Acrylic paint. The first color I used was Raw Siena which is a very dark brownish yellow. You can see this color at the very edge if the rusty areas. The second color was Burnt Siena for that redish rust. The transition from Raw to Burnt Siena is easy to see in the upper left quadrant above the slope sheet in the first image. The last color is Burnt Umber which is a very dark brown. I like Burnt Umber for old pitted rust. All these colors blend together for what I think is a convincing rusty appearance.
For the couplers I apply the colors in reverse order, By using the Raw Siena last I can use it to highlight the edges and detail in the coupler and also create a dusty/rusty look.