The vast majority of my painting and topcoating is done via airbrush, but I keep some spray cans on hand for large pieces and special cases.
I don't know much about painting cars, but I'd venture a guess that procedures are well-documented to minimize mistakes. While you can find plenty of info online for model painting, the procedure and technique can change depending on a variety of factors (products used, painting method, temperature, humidity, etc). What brand/type of topcoat are you using? When in doubt, it's best to test on a spoon or piece of scrap runner before painting the model.