Speaking from experience, I wouldn't put a car cover on a car stored outdoors, especially if it's in good shape. No matter how good of a job you do securing it, the cover will get moved around somewhat on the car on very windy days. The problem with this is airborne dirt, dust, etc. particles make their way underneath the cover and then the cover rubs those particles against the paint abrading it to some degree. The only way I would consider a cover these days is on a car that's indoors and cleaned very well time before the cover is installed.