I am building an arched valance for our kitchen window in front of the sink. The bottom rail provides the arch. Simply for stability, the rails and stiles are 3/4" PVC, and the two raised panels will be MDF. I've managed so far to layout the curved rail on the PVC and rough cut it. I've also smoothed the top side of the arch down to the layout line using primarily my belt sander but also some handwork. Now the question is how to smooth the underside of the arch, the "inside" or concave portion. The radius of this curve is about 6'6", (the arc itself is about 5' wide), gradual, but not so gradual that I can put the belt sander to it and not risk gouging places to the rear of the belt or digging in too much in the front. Anyone have any suggestions how to smooth this curve? It would be the same challenge if built from wood.