Ground as in electrical ground, or ground as in ground plane/SWR optimization?
Its very hard to optimize the ground plane with the kind of mount you have, because most of the antenna length is along the side of the rig, and, the vertical part that clears the roof is very short, and, at an off angle, etc.
If its an electrical ground, then, sure, sanding to bare metal and/or installing an added ground strap, etc, can help.