The role of a fairing board is to knock down high spots. Any tendency to sand in low spots just means you need to bring the high spots down farther. You don't need or want it to conform to small irregularities.

The elevation differences between the high and low areas you are trying to eliminate are on the order of few thousand of an inch. These highs and lows are such that you don't really see or feel them until you have varnish on the boat where the reflections of straight lines get rippled due to slight changes in the surface angle.

With that in mind little or no paddling will make the most effective fairing board. Lack of paddling can be hard on the sandpaper, but you don't need to use a lot of pressure to do effective sanding if you have sharp sandpaper.