Before a rough board can be planed to thickness, you must be sure one side is flat. Wood is flexible, so if you send a warped board though a planer, the feed rollers will flatten it and the planer will plane it, but when it comes out it will spring back, and you'll still have a warped board. If one side is not already flat, you will need to plane off enough wood from just the highest spots so that the board will not deform under pressure. You do not need to plane the board completely smooth at this point, hence the term "skip planing."
Things You'll Need
Decide which side of the board will be facedown when you feed it through your thickness planer. This is the side you will skip plane.
Check for warp. Place a ruler across the width of the board to see if it is cupped. Place a long straightedge along the length of the board to see if it is bowed. Place a ruler across the width on each end of the board to see it it is twisted. The places that the ruler touches when it doesn't lie flush against the board are the areas that need to be planed off.
Plane off the high spots with a plane that is long enough to ride over the low spots without cutting them. In this example, you can see the hollow side of a cupped board. The plane removes wood from the edges, which are higher than the middle. Notice how the plane is held on an angle so that it will skip over the middle of the board.
Stop planing when you are satisfied that enough stock has been removed to prevent the board from deforming as it is pushed through the thickness planer.
Place the side that you have skip planed facedown on the table of your planer and plane the board until the upper side is clean.
Turn the board over and plane the other side to the final thickness.
Place the board on your jointer with the side to be skip planed face down. The jointer must be wide enough for the face of the board.
Place your "push paddles" only on the places that already touch the table. Use a light touch. Do not squash the board flat against the table
Make several light passes across the jointer until most of the board is supported by the table without being pressed down. This picture shows a board that has been sufficiently skip planed.
Follow steps 5 and 6 under "Skip Planing by Hand."
Always wear safety glasses and hearing protection when working with jointers and planers.