When a piece of wood is “cupped,” it means that the edges of the board are slightly raised in the center. This can happen for a variety of reasons, but most often it is due to changes in moisture levels. If you have cupped wood, there are a few ways to try and fix it.
One way is to use weights or clamps to flatten out the board. Place the weights or clamps on the high points of the cup (the edges) and leave them for a few days. The weight will help to push down the raised areas and flatten out the board.
Another way to fix cupped wood is by sanding it down. You’ll want to use a belt sander for this, as it will make quick work of removing any raised areas. Start with a coarse grit sandpaper and work your way up to a finer grit until the surface is smooth.
If your cupped wood is too far gone or you can’t get it flat using either of these methods, you may need to replace the board entirely. Fortunately, if this is just one board in a larger project, it shouldn’t be too difficult to match up another piece of wood that will fit in its place.