Once the boards have been de-nailed, they are then resawn and surfaced to a uniform thickness. Now they are ready to laminate to a 1/2" sheet of birch plywood. I try and use a stain grade plywood any time I use this procedure. It typically lies a little flatter and is free of voids. After the material is dry, and fit into place, I mix epoxy with a powerful dye to tint the batch and hide the bright white color of the plywood that will show thru the knot holes.