Sanding curved parts with power sanding equipment can be nearly impossible. Instead, I like to turn to hand sanding for smoothing these parts. To aid in sanding these curvy pieces, I always keep one of these shop-made detail sanders on hand.
I size my detail sanders for use with 1" x 30" sanding belts, but other sizes will also work. The important thing to remember is the shape of the sander. They are designed so that when gripped near the curved end, the belt is properly tensioned to stay in place. I usually cut them a little oversized and sand them down to fit.