After trimming the ribs with the chisel, I used a rasp to smooth out and level the edge. It should be perfectly straight.
I used a long, thick straight edge to check for high and low spots and warping. It has to be level everywhere: top, bottom, sides, and diagonally. After the board is level, I cut large pieces of sandpaper (from sanding belts) to fit the board. I used coarser grain on one side and finer grain on the other side. The sandpaper was attached to the board using spray adhesive.