If I run the top face of that board through a planer, the planer will act as though it's flat. How do I get a board that way? I'll explain how as we go along. Here's how my weekend went:
- Check with winding sticks in three positions: a foot away from each other at the center of the board, halfway between there and the ends, and then right near the ends. Usually, you find that the twist increases as you approach the ends of the board.
- Make no more than three passes of the electric planer (or jack plane, or disc sander, etc.) on each end of the board, obviously attacking the high side of the board only. The first pass is only on the foot nearest the end, the next pass is only on the two feet nearest the end, and the third pass is on the three feet nearest the end. Notice how that means you're taking three times as much from the end of the board as from the center?
- Use the winding sticks to check your work. Hopefully, the winding sticks are closer to level than they were a while ago. Sometimes, the winding sticks show that you've lost the concave shape across the width:You can see the pencil marks where the winding stick is touching a high point. I use the pencil as a guide to show me where to re-excavate my lengthwise trench, so that I can check for twist with the winding sticks again.
- Use a long straightedge or a string (or your eye if you're well-practiced) to make sure the board is still slightly concave along its length. If you need to, plane away the full width of the center of the board until the entire face is concave along its length again.
- Repeat steps 1 through 4 until you have a board that doesn't twist, is very slightly concave along its length, and is concave across its width.