[donv]

If you're planning for storing the car in a garage, a soft cotton flannel cover (essentially just dust/scratch protection) is a safe bet and one of your least expensive options. For rain exposure, etc. there are other options including the "Noah" fabric, etc. I have a Noah cover which I use very frequently and have noticed no scratching. For the most part, it would be more the dirt you may get trapped under the cover rather than the fabric it is made of that would contribute to scratching. To that end, a cover that is machine washable (like the cotton flannel) would be the most convenient.