I have a project that is basically a very thick slab of wood( 6" thick) with 4 squares holes(they don't go all the way through). The holes/slots/mortises are very deep.(4" deep) I'm not going to put a tenon or anything else inside these holes, so the walls and the bases of holes
(see image 1, the base is marked will purple lines) will be visible.
Is there a tool(see the 2.image) that can go in these holes and flatten/smoothen these bases?
I'm interested in hand tools mostly. Sorry for the low quality paint.net images :-)