The feed rollers on a planer press down so hard, they only temporarily flatten a cupped board.

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The feed rollers on a planer press down so hard, they only temporarily flatten a cupped board. So instead of a flattened board, you end up with one that's thinner, but still cupped. To truly flatten cupped boards too wide to face-joint with a jointer, shim under the cupped face using veneer.

Stack 1"-wide strips of scrap veneer to fill the gap, as shown. After planing the top of the workpiece flat, remove the veneer, flip the board, and plane the opposite face as you normally would.
—Wayne Burton, Stoney Creek, Ont.