One important freight car location where rust is sure to appear is inside steel gondolas. Gondolas can take a lot of abuse in loading and unloading, and often side sheets, ends, top chords and floors all show dents and scrapes. These naturally damage the paint layer and allow access of moisture in the environment to reach the steel material of the car body, and pretty soon you have rust.
Here again, as explained in the first post on this topic (link provided above), the sequence as time passes is yellow rust, red-orange rust, and last, brown rust. Portrayal of rust on freight cars thus requires the modeler to decide whether it is old or new rust, or a combination. For these colors, I use artist’s acrylics. For dark brown, I use Burnt Umber; the reddish color is Burnt Sienna, and the yellow is Raw Sienna. These can be applied as either a wash or in blotchy patches. Since they are water-based, you can simply rinse off any effects that don’t go the way you want (I’ve done that many times), and try again. Just remember to do the rinse fairly promptly, since once the paint sets up, it is no longer very sensitive to water. And acrylics do dry quickly.
For a first example, here is the EJ&E gondola I mentioned in a previous post about route card boards. If you wish, you can see that prior post at: http://modelingthesp.blogspot.com/2011/12/route-cards-5.html . It is a modifed Athearn gondola, with a wood floor (scribed styrene) and rivet-impressed styrene liners to provide the interior rivets on the side walls. I have applied general rust colors, along with linear rust marks to indicate scrapes or scratches of the interior. This photo is from the left side of the car. Note that some rust staining has extended onto the floor.
From the right side of the car, the other interior side can be seen. In this view, some scraps of wood dunnage can be seen, which were left from the previous load.
Another technique, especially appropriate for cars that often carry bulk cargoes, such as drop-bottom or GS gondolas, and open-top hoppers, is the yellowish rust that forms on relatively clean steel. Every time a load of coal, sand, ballast or other such cargo is loaded, then unloaded, much of the interior, especially floors or slope sheets, is scoured clean of dirt and old rust. In the presence of moisture, that steel surface will again start to rust with a yellow film. Here is a GS gondola model which attempts to depict this situation. This SP Class G-50-15 car is from a Detail Associates kit.
Heavier rust, as old paint damage areas continue to darken with time, is shown by dark brown colors only. Here is another SP gondola, Class G-50-13, built from a Speedwitch resin kit, showing that appearance.
These are some of my efforts to show heavier rust areas, and I am always trying anew to do this better. As with so many modeling topics, a big benefit is to look at the prototype, get photos if possible, and try and duplicate what you see.