It's kind of interesting, that I was once told by a car paint guy that a build up of dirt in the winter, if you don't keep it washed all the time, is actually better for the car than a semi clean. the reason for this is that the dirt acts as a barrier to the salt and the moisture and therefore cuts down on the oxidation overall. The paint is alot tougher than you think, and too much washing lends itself to water staying in recesses and the like and hastening the rusting process. That's why heated garages in the winter, while more human pleasing is not all that good for the car, it keeps the water in liquid form while at the same time mixing with salt. Better to let it freeze dry and slowly evaporate. I take mine through the wash about once a week, and hope for the best.