I have a few tricks to make rust.
1 - I use A.I.M powders with Micro-sol and a very small brush, Take the brush and dip it into the Micro-sol and then dab it into the powder then stiple it on to the side of the car.
2 - Gouache waterbased paint in a tube, This type of paint has a texture to it and when dry looks like a very fine coatting of rust.
3 - Hairspray the type in the pump bottle, Paint or color your then spary a fine even cote of hairspray on then sprinkle on your powder or chalk and let it dry then give it a shot of Dull-cote.
4 - Items 1 - 2 can be mixed together to get some really nice results.
Here is something I am working on it may give you some ideas?
http://www.modeltrainsweathered.com/forum/viewtopic.php?t=547