I am venturing into a new area, so I am lacking in in a few things. I am experimenting with making some shallow platters. Starting 4/4 wood. Platters 6-8" in diameter. I seem to be able to get forms and shapes I want but finishing has become a challenge for me. When I am sanding, I can't get rid of the bumps in the bottom. I think the variations in the grain are the issue. Some of the grain seems harder than others and as a result sanding varies across the grain or with it. Any suggestions or approaches?