Weathering Your Car’s Roof

Most of us spend an awful lot of time looking at our model freight car roofs than the actual model itself. So, basically, it just follows that we spent extra time weathering these areas. It’s true that although some real boxcars have roofs that are painted, most have none. And most of the time, paint sprays from the car can be seen all over the roof’s edges.

http://modeltrains.about.com/b/2011/10/12/cool-trick-modeling-paint-overspray.htm