Ya, Deva, after adzing, hand planed and then sanded. I see some guys initially shaping seats using something like a hand grinder that has like a chainsaw chain cutter head on it, then they refine the results from there. I have never used one, but I guess they work pretty efficiently for roughing out. It might be something to look into if you wanted to shape seats. Hand planing has it's merits, but a slower process and there is the learning curve. Nothing wrong with a straight bench, but shaping to fit the butt does make for more comfortable seating than simply sitting on a flat board, by a long shot. Also you get the sculptural effect. If I was making that bench now, I probably wouldn't sand it, generally prefer to leave tool marks as long as the right shape has been accomplished.