How to Remove a Cup From a Board? Add some moisture to the concave side because the cup is likely the result of incorrect storage or drying. But, if the grain on these pieces is extremely irregular, you may be dealing with reaction wood, which is more difficult to work with.
I will presume, for the purpose of advice, that this issue is due to unequal moisture content rather than reactive wood. Most likely, the pieces were stored so that the concave side was exposed to air/sunlight/heat, causing it to dry faster than the opposite side. It is possible that these were the top parts of four stacks of treads that hung around for some time, allowing for uneven drying, or that they all originated from a single board that was stored in this manner before being cut into four pieces.
Moisture can be introduced in a variety of ways; for the time being, all that matters is that the moisture content is uniform across the thickness of the board in order to restore its flatness. The most efficient method would be to reintroduce the moisture by steaming it back in, as this will accelerate the penetrating process. You can use a wallpaper steamer or a teapot on the stove for this purpose. Put on some heavy gloves and play the concave side of the tread back and forth in the steam plume for a few minutes, then set it aside while the first one absorbs more moisture into the board. Rotate the pieces in this manner until they all appear somewhat flat, and then clamp them so they remain in this position for at least two to three days.
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This time, they must be secured such that airflow is uniform from all sides. Place flat & true stickers approximately every 10″ or so between the pieces, as well as on the bottom, before stacking and clamping them. You are aiming for uniform airflow and constraining force, while the components balance out their moisture content throughout their thickness. Then they must become flat again. This should be performed in a moderately-temperatured environment.
We can manage the rate of moisture exchange by selecting the type of finish and number of layers. For example, we want to apply the same amount of finish to the bottom and top of a solid-wood dining room table. Without it, the tabletop will cup when the relative humidity of the surrounding environment fluctuates.
Ensure that the “excess” moisture has evaporated from the steamed sides of the boards before applying a finish; otherwise, it could result in finish failure. I would recommend equal quantities of finish on both sides.
When clamping these boards, make sure the clamps are secure, but not so tight that the board cannot move due to fluctuating moisture content; otherwise, it could crack. As it seeks equilibrium, the wood must have the ability to glide between the stickers.