Unlike the outside where sanding is easy, the inside can be harder to to smooth. I find scrapers are the easiest way to deal with the concave surfaces. A set of scrapers with a variety of different blade profiles can make relatively quick work of the task. I have a selection of different scrapers that I have modified with different shapes to fit the various contours of the boat.
After scraping I wrapped a chunk of foam with 40 grit sandpaper to further level the surface. A few more touch-ups with scraping and more sanding with 60 grit prepared the surface for carbon fiber.
Since I am using carbon fiber fabric which is opaque, I do not need to achieve a cosmetic finish, coarse sandpaper is enough. The rougher surface makes a better bond with the resin.