This is how I am using the longboard sanding tool I bought from Jamestown Distributors. It is a tool normally used on boat hulls, and it is nearly perfect for this, too. The swirling motion and figure 8 motion tend to avoid any gouges or divots.
I was able to even up the surface so that the several low areas are largely gone.
I wish that my video camera had a wide angle lens - I had to position it quite far away, and I hope things so up clearly enough.