Over the years I have tried many different methods for sanding panels such as yours and I still revert back to what I did years ago while working on vintage cars everyday.
This is an odd concept here, stay with me.
I look at the curved panel as what I call a "Curved flat panel". I treat it the same as I would a flat panel, only I sand just a very small portion of it at a time. I feel for a high spot and sand it down, then find another high spot (usually VERY close to where you were just working) and sand it down. A slight "roll" on the block, but basically that's it. I use a regular 5" rubber block usually.
The trick being LOTS of feeling and VERY LITTLE sanding. Sand the high spot very carefully and lightly, then feel it. Over and over, until the whole area is done. I know it sounds odd, but I am darn sure I could sand a block of filler into a bowling ball if I wanted to using this method.
A while ago I did a set of 72 VW Bug fenders using this method and they turned out killer.