One other thing of note before I explain my weathering technique is that these cars come ready to run with body mounted MT couplers. While I was surprised to see this and concerned about the length of the car, it operates flawlessly on my 15" mainline curves, even when coupled to shorter rollingstock.
Here is the SP version which I will begin soon. It will get a heavy fade and lots of rust. The UP version (not pictured) is a fairly fresh repaint and will get a light fade only. It already has the safety striping which is a plus.
Here is the SSW Cotton Belt boxcar right out of the package. Even when it was new, it was an ugly beast in my opinion.
The first thing I did was take off some lettering with a small piece of sand paper. A nail file works well for this too. I roughed up the large "SP" to match the wear and bleeding of the white seen on the prototype today.
I used a very heavy fade on this car to dull the red paint. It turned the car pink, but that will be corrected in later steps. Next I added some brown and rust colored chalks to the grey stripes. I was hoping that the grey was painted on top of the red, but it is not. If it was, I would have rubbed away some of the grey color to expose the red below. On the prototype, the grey gets worn and thin, letting the red show through underneath. Instead I used chalk to replicate this.
After a coat of Testors dullcoat, I painted the entire car with slight thinned burnt sienna oil. I rubbed the excess color off using a q-tip and small paper towel pieces. The color is starting to look correct now. Let the body sit for a day or so before moving on.
Here is the finished car after applying burnt umber over the previous step. I'm not yet finished with this car, but this is about 95% complete. I added safety stripes, painted the brake wheel platforms silver, and painted the trucks with Polly S grimy black followed by some chalks. The other side still needs safety stripes as you will see in later photos.
Overall I'm happy with how it turned out. I don't want all of my rollingstock to look like this, but having a few crusty pieces in the mix is always good. It makes for a more authentic fleet.