I'm a fairly new handtool woodworker and I'm building a Nakashima style bench as my first project. (I'm basically copying the George Nakashima's R bench.) I have a large 8/4 slab that my wife and I recovered from her grandparents' Pennsylvania farm about 20 years ago. I think it's oak, but I'm not sure. It's about 8 feet long and tapers from 14 to 16 inches in width. I plan to crosscut the wide end to make a leg for one end of the bench.
The 2-inch thick slab has a 3/8-inch cup and I'd like to flatten the top and leave the bottom the way it is. Both sides are currently quite rough. I can't decide whether to plane the concave side or the convex side for the top. I'm leaning towards flattening the convex side and making that the top. Are there any advantages or drawbacks for choosing this approach? I'd like to avoid causing more cupping or perhaps encourage the non-planed side to be more flat.
— Robert in Albuquerque