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wash Washes...
One way to make your models look like they have been used for a while is to use a wash.  A wash is simply thinned paint that is applied with a brush to the car.  The thinned paint settles into the cracks and crevices just like dirt does.  This is also used to add depth to a car.  By making the cracks a dark color they appear deeper.
Below is a pic of how dirt and grime accumulates in the crevices.


How to:
To do a wash simply thin a paint about 1 part paint to 8 parts thinner.  Apply it with a large brush in vertical strokes on the car.  This will leave a streaking on the car, very similar to the real thing.  If you don't like the streaked look, simply use an eraser at the end of a pencil to erase the thinned paint.  I used this technique on the covered hopper below.  It leaves a look that is very characteristic of a railroad car.

The white areas on the car depict spills.  I will cover this technique in the "streaking" page.