Car covers are used to protect vehicles from damage caused by nature. Storms, tree sap and damaging ultraviolet rays from sunlight are common concerns treated with a car cover. Wet cars should not be covered because it can cause more problems than it prevents.
Car covers prevent interior fading caused by sunlight but they also prevent sunlight from drying the car more quickly. Water left underneath a car cover for long periods can lead to mold.
Dirt often causes scratches on cars. If water on the car contains dirt or debris, spreading the cover over the car can rub it in and scratch the paint.
Moisture trapped under the car cover can leave water spots on the vehicle. If the moisture is from using tap water to wash the car, the additives in the water could leave permanent spots.
The outside of a car cover is usually waterproof but the inner lining generally is not. The water transferred from the wet car to the cover will take a long time to dry. This allows problems like mold to persist long after the car itself is dry.
