Rust is easy, just buy some rust paint! For example, look at this exhaust system I did using Floquil "rust" color, sold through their railroad paint line...
All I did was airbrush rust onto the bare plastic. After that was dry I did a light black wash followed with a gray wash. Then when that dried I did a light drybrushing of silver to simulate ground scrapes.
For the body I sprayed Silver, then rust, then covered the whole thing in "grimy black" (also Floquil). Depending on how hard I scraped with an xacto knife or sanded with sandpaper it would expose the rust or "metal". Some parts I would dig out with a blade, dab some superglue and baking soda on it for texture then brush with rust paint. The baking soda gives it that cobbly rusty texture. If you want a thin rusted section, grind from the inside with a Dremel or something similar.
Rust isn't everything, though. You need to do the whole package for a convincing look. This model was done with rust, black, grimy black, mud, dust, grime and I forget what else. The seats and top part of the car was given a light white drybrushing to simulate that sunbleached look. The corners that would normally see wear were sanded, drybrushed, and show slightly more rust if made of metal. The parts exposed to the road were sprayed with grime, mud, and dust at very low pressures so they look "splattered" on rather than airbrushed on.
Here's the rest of the model.
http://public.fotki.com/lauron/rons/..._ford_phaeton/
Get a set of Floquil's "railroad weathering system" which comes with a bunch of weathering colors then experiment away. My '32 for example was my first ever weathering experiment and the paints make it easy. Just remember not to overdo anything; subtlety is best when weathering.