The area between the fingers in the photo on the left is marked by "snipe." These mill marks have been removed with a handplane in the section in the right photo. When using a sanding block, hold the block flat with constant pressure. Don't let more than half of it overhang an edge.
Application of a finish to the inside faces of a project is best completed before glue-up because it would be a challenge to work on these areas once the project is assembled.

Generally speaking, once a subassembly is glued together it will be more difficult to get at the inside faces with a plane or sanding tool than when it's in pieces. For this reason, it's a good idea to "clean up" and "polish" all those faces of each piece before assembly. "Cleaning up" is the process of preparing the surface for the application of polish. "Polishing" involves applying a finish, such as shellac and wax, to the pieces.

Cleanup entails the removal of machine marks and machine tearout, which on a flat surface will have come from a jointer or planer. A hand plane is a good tool for this task because it's essential to retain the flat, straight and accurate machined face.

After the surface is smooth, it's time to apply a finish to the inside faces. One quick and easy combination is to apply shellac followed by wax. This combination has the added benefit of retaining or even enhancing the color of the wood. Another big payoff is that prefinished surfaces resist glue penetration, so you won't have to rush to wipe up any squeeze-out. Let it dry, then insert a sharp, wide chisel under the edge and the excess glue will easily release, leaving a clean corner or face. Of course, be careful to keep the finishing material off all joint faces because it will interfere with the glue's ability to hold the joint together.