Drybrushing

If you want to weather your cars or buildings I suggest using a technique called dry brushing.  Dry brushing is easy.  Here are the necessary tools to make rust streaks, Paper towels, Orange paint, Brown paint, paint brush.

1. Start by miking one part brown paint to 10 parts orange paint.
2. Lightly dip the brush into the mixture of paint.
3. Wipe the paint of with a paper towel. 
4. Gently rub the paint brush across the surface you are weathering

Why this works.

This method makes rust streaks because  when you wipe the paint off of the brush and onto the paper towel most of the paint comes off so when you brush the surface you are weathering the brush applies a very thin layer to the surface instead of a thick layer

-The Model Railroad 101 Team