A bit involved but maybe worth trying.
You are trying to build up a smooth epoxy layer. What if the epoxy hardened in an absolutely smooth surface that required little or no sanding?
I've had some luck with this method on smaller repairs.
Get a mold release agent McLube : Mold Release
and some panels of plexiglass that will fit over the flat sections you are coating. Lay the epoxy on as evenly as possible then spray the mold release on one side of the plexi and lay the plexi down on the epoxy. Use some weights if you have to so the plexi sits flat and comfortably while covering the epoxy. Once the epoxy hardens you should be able to remove the plexi (hopefully in one piece) leaving a smooth surface that needs little or no sanding.
This technique works pretty well with Bondo as well.
Other plastic panels should also work for making mold shapes. I've used thin plywood (1/16") so it could be bent to match the shape of a hull but it is hard to get all the ply off the epoxy once hardened which leaves you with some sanding to do.