Finally sanded the wavefront to desired shape. There were some very fine gaps, in certain places, between the layers of Masonite. I went over those with some wood putty (tan colored), and sanded them down. Then I sanded the whole surface for preperation for painting.
In all honesty, it became a task I do not wish to repeat ever again. Maybe when I was younger, but sanding for endless hours was very straining on my hands and arms. I am not used to it.
Here is a photo of the support just before vacuuming the dust for the final time. Actually, dust went everywhere. I can see I will have to clean all my painting eqipment and supplies before I start painting.