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It is best to wash the car on a day that is not too hot, out of direct sunlight, and avoiding windy conditions. If the car body is hot, shampoo may dry out quickly and cause staining. In any case, be sure to rinse each panel before moving on to the next, to ensure the shampoo does not dry on the surface. Hot conditions can also lead to unevenness in waxing and coatings.
Windy conditions can bring sand and dust onto the paint during washing, leading to scratches when drying the car.