Dont expect the rustoleum to be thick enough to cover any surface issues up! especially high spots from old artwork. Spend time sanding to get the surface very smooth, or it will definately show.

what grit are you using and what type of block? I would think 100 or 120 would be a good start for the artwork without doing too much extra damage to the surrounding paint, then go over it with 180 or 220 to get rid of sanding scratches before painting.