First and foremost, never, ever, use dishwashing soap to wash your car. Dish washing detergent strips off wax and can dull the finish. Use either a high-quality car wash product available at leading retail and auto parts stores or make your own. See the recipe box section of our site for how to make your own eco-friendly, cost effective and safe car wash and other car cleaners. Second, never wash a car when the body is hot or when the car has sat in direct sunlight for long periods of time.