Weathering Roofless Boxcars

I started to weather my roofless boxcars yesterday. They were airbrushed with thinned sand color to fade the paint and thinned brown/black to represent accumulation of dirt on the lower parts. Then, several effects were obtain using oil paint and mineral spirit. A very classic way to weather but that gives great realistic results.

When everything will be dry, I'll touch down the model with pastel chalk and paint the wheels.