I recommend using the blue soft sander blocks for this next step. I would not use durablocks because they are far too stiff, unless a panel is really flat. This body appears to curve in all directions, much like the my '37.
I really prefer super build when trying to build up some thickness.
This type of primer will always have a fair amount of orange peel, but that helps when sanding. You can watch it disappear and then stop sanding. If you see the gel coat showing, definitely stop, since it sands much slower than the primer.