I'm in the process of repainting the back of my 9800 too (funny coincidence: I bought the exact same primer and paint as Rene without having seen the pictures from his above link).
Removing the old paint is fairly easy with 1000 grit wet sanding paper.
You need 1 sheet and about 1 hour time.
At first I tried rubbing off the paint with the coarse side of a Glitzi sponge, but that didn't work well and I feared that it would leave scratches.

Sebastian