Car covers dont always do the trick, and can make the problem worse. It gives a cat something to really claw into, and their claws are much longer than the cover is deep. Plus, if a cat really takes a liking to the cover they may claw it up which will also result in damaged paint. The only sure way to prevent claw marks is to keep the car indoors, especially at night when most cats are active. If you can't keep it indoors and try the cover route, get a thick cover. Another option is to buy one of those car tents which is sort of a cloth garage. You can enclose them on all sides.