Auto carriers and high cube cars weathered using oil wash (from an article by Ken Patterson in 2005 from Railroad Modeler (now out of production). An airbrush wash was added to the lower areas to simulate dust and grime.  Chalk and pigment powders were use to add depth.

The CNW car has one coat each of very thin dark grey and burnt sienna.  The roof and side areas immediately below were given a more concentrated layer of the sienna to depict the beginnings of rust and subsequent rain driven stain down the sides.

This UP car is one coat of very thinned grey and highlights using 2 layers of burnt sienna to simulate rust on the roof, down the side, and in the door area.

Same as before but with and more pronounced rust effect on the sides.

Here's a line up of a few of the auto carriers.  All have since received additional weathering via chalk and gouache.