I share the frustration of scanning watercolors with less than desired results. One thing I've noticed that affects results is the angle of the paper on the scanner. This image shows the difference between scanning at a horizontal angle and a verticle angle. The first image has the grain parallel to the fluorescent bulb itself, and perpendicular to the path of the bulb. (Click to enlarge and see the difference.)
I think this is the trick with photographing art as well; to set up lighting so that no shadows, at least unflaterring ones, are created.