Before you use a car cover your car has to be spotless. Not a speck of dirt on it. Any dirt on the car when the cover goes on, and it becomes trapped between the cover and your paint. As you put on the cover, and take it off, as well as when the wind blows or rain falls the cover moves and the dirt becomes like sandpaper, marring the paintwork you are trying to protect. trying to use an external cover without affecting your paintwork in some way is almost impossible.
This is why i never use covers. i rely on many coats of the most durable waxes/sealants and parking my car away from trees.