My "Help"...
There have been a few articles over the years about using a PC etching acid solution to create very realistic weathering and rusting on this Campbell stuff.
One of the best, in my opinion, was by Rand Hood, (an HO Rio Grande guy) in Model Railroader magazine, but I forget the issue.
Essentially, the method is to purchase PC etching solution from Radio Shack, and just dip the pieces in for short periods of time, and then dip them into a baking soda solution to stop the etching process. Rand showed how to weather panels, and how to make and weather culverts. His results were spectacular.
This is, however, an
extremely dangerous process. There are noxious, acidic fumes, as well as the risk of fingers and what-have-you getting burned. (drips and drops on the table, your pants, shirt, the carpet, etc.) Also, all containers have to be
THROWN OUT AFTER USE, mostly to avoid being mistakenly used for human consumption. Then, of course...what to do with the acid once you're finished your project?
The closest I ever came to working up the nerve for this project was to ask my local Radio Shack how much the etching acid cost. They had no idea what I was talking about.

Maybe it was all for the best.