Your pallet board may not be worth the effort, but there are several techniques that you can try to salvage any warped board. The optimal solution will vary depending on how you intend to use the board.
Wikipedia has a nice primer on specific types of defects in wood which fall under the blanket term of warping:
- bow : a warp along the length of the face of the wood
- crook: (also called wain) a warp along the length of the edge of the wood
- kink: a localized crook, often due to a knot
- cup: a warp across the width of the face, in which the edges are higher or lower than the center of the wood
- twist: a distortion in which the two ends do not lie on the same plane
Your strategy for dealing with a warp will vary depending on the specific type and severity of the defect. For less severe defects, you may be able to simply make the lumber square, flat, and straight.
In the best-case scenario, you joint one face and edge on the jointer, then rip the other edge parallel on the table saw and flatten the other face parallel with the first on the planer.
However, at each step along the way, you necessarily sacrifice some of the wood's thickness and/or width.
A board could be so badly bowed or twisted that there wouldn't be anything left if you tried to joint it straight. But if the components of your project require shorter boards anyway, you can crosscut a bowed or twisted board into shorter pieces, each of which may be flat enough that it does not require much jointing and planing to make it flat and straight.
Another option with a bowed board is to either resaw it or cut it into shorter sections (depending on whether length or thickness is more important, respectively), laminate the pieces (glue them together, one on top another), then joint and plane them. Just be aware of any internal stresses that you may be adding to the finished board, and try to avoid a glue-up that requires massive amounts of clamping force to close up gaps between the boards.
If you have a board with a crook or kink, you may be able to rip one edge straight, then trim and glue the offcuts onto the opposite side.
If your board is cupped, you have multiple options depending on whether it's more important to preserve the thickness or width of the board.
- If the cup is fairly shallow, you can joint and plane it.
- If the cup is severe and simply jointing one face and planing the other parallel would make the board too thin, you can rip it into strips first. Each strip will have a less severe cup then the original board, so you can joint and plane the faces of each strip individually, then joint the edges and glue the strips back together into a panel.
If you have some combination of defects, you may have to employ several strategies to get workable pieces of stock out of a warped board. Whichever strategies you use, make sure you are practicing safe techniques. Also beware of any internal stresses that may be released as you cut a warped board, as they can result in binding, fragmentation, and/or kickback depending on the tool.
Again, some defects are easier to remedy than others, and even once you've remedied the problem it may return depending on the properties of the board. It's good to be familiar with the differences between quartersawn, plainsawn, and riftsawn boards and each type of cut's inherent movement properties.