Originally Posted by rgrwilco
The bottom of the rack sack is made of a much softer material than the rest of the bag. It's made to be on your roof without damaging the paint. A film wouldn't hurt but it's not necessary.
Does the sack contact the roof at all?
With any dirt between the rack sack and the paint, not so good.
With fluttering caused by driving faster than 12 miles per hour, even worse.