You should wash your vehicle when it is dirty, regardless of whether it’s before or after a rain. We know what you’re thinking, “yeah right, they’re just saying this because they’re a car wash”. Nope, this is a fact based on real science and years of experience among auto dealerships that need to keep their inventory looking perfect while parked outside. When your car is dirty, the surface holds all sorts of dirt particles, contaminants and road grime. Now just add water and you end up with cement. New paint problems are born when water-soluble solids mix with rain. So rain will not clean these particles off of your vehicle. It just mixes with the goo that ruins your paint finish.