I have alway had a problem finding a backing for sand paper when sanding concave and convex surfaces which I find a lot in working with old cars. I have used radiator hoses and flexible rubber pieces but I think I finally found the solution. I had a lot of mouse pads and wrist rests laying around last night and needed to do some wet sanding on some wheel rims. If you take a mouse pad or if you want a thick pad a wrist pad and cut it. It makes a great backing pad. One side of the pad is usually textured and grips the paper. The other side is smooth. Depending upon the pad you can get a stiff pad or a soft pad and because it is a rubber based you can use it in water. If you want to contour it just use a knife and some 80 grit paper and you can make it any shape you want. I also use drip irrigation hose held by small suction cups to provide a constant water source on large flat surfaces. Hope some one can use this idea and improve on it.