I've used a clay bar many times on my cars. Although horizontal surfaces are usually rougher, the rear hatch area of any car will always be rough. The key to using a clay bar is to be gentle. It's an abrasive, and it needs to be run lightly across the paint, with plenty of the proper lubricant, not water.
I use Mother's clay bar kit, but there are other good brands out there as well.
Wash the car thoroughly, dry it, use the clay bar with the recommended lube, wash the car again, and then wax it.
I usually don't have to do this more than once a year, or maybe twice on an older car.
It's worth the effort.