I like to call this technique the Shake and Bake method. What you do is take a powder such as tile grout or aquarium filter carbon or some other suitable color and put it into a gallon size freezer bag. Add you item to be weathered such as a piece of rolling stock or a loco shell. And roll the item around in the powder. Open the bag and remove the excess from the nooks and crannies and either seal with dull coat or apply other colors of weathering powder to it. It works very well as the following photos will show.