Originally Posted by
FLBri
Sandpaper will be more controllable on an irregular shape like that. Depending on the amount you want to remove, I would start with 220 and progress to 400 with one or to steps between. Try to keep your removal to the very edges, because if/when you hit the back flatter area you will change the sheen.
I would put some masking tape, or electricians tape on the flats that you don't want to change. Finish the edges off with a gray scotch brite pad and you should be pretty happy with the result.