There seems to be a lot of interest here in weathering freight cars and locos. Weathering adds to a model's realism. There are several methods to achieve a weathered look such as airbrushing, washes, etc. The method described here works well for me but keep in mind, there's a right way, a wrong way, and there's grande man's way. [:D] This is a quick and dirty (no pun intended [:D]) method to turn out a quantity of rolling stock in a hurry. It probably takes about 10 minutes per car plus drying time for the dull cote. Maybe some of ya'll will find it useful.
The "victim" and basic materials. The chalk came from a LHS and Testors Dull Cote is widely available. I prefer an angled brush because it can reach into small spaces yet still cover larger areas. I use the same brush for all chalk colors.
The first step is to add soot. Go ahead and coat darker colored cars like this oxide color fairly heavy. For lighter color cars add a little less "soot".
With a paper towel dampened with "wet water" (water with a few drops of dish detergent added for spraying scenery before cementing it) add streaks to the soot. On a car body such as this hopper, I like to leave soot around the verticle structure.
The next step is to apply a coat of Testors dull cote over the entire model. This seals the soot and "grabs" chalk added later. Don't get dull cote in the truck journals. Tape them if necessary.
While the dull cote dries, apply a cote of Floquil Rail Brown to the wheels. Be careful not to get paint in the journals or on the treads.
Add "dirt" with appropriately colored chalk.
Complete the final details like adding dirt and rust colored chalk to the trucks, couplers, etc.
Viola! The "dirty" hopper at home in the dusty, soot filled environment of the Split Rock Mine.