Occasionally you need to make a piece of warped foam board straight and usable. For me it is the horizontal and vertical stabilizers, rudders, and elevators. Not so much a problem with fuselages and other parts that have panels that are bent and glued. Those tend to automatically straighten out any warps. So I usually take 2 warped pieces at the same time. Put a couple of strips of foam board under each end, then add some sand bags in the middle. If you get too aggressive with lifting the ends and adding sand bags you will end up putting wrinkles in the paper covering right where the sand bags sit. It normally takes a few to several days for the boards to straighten out. These pictures were taken outside in Chicago on a cold day. The warmth of being inside the house will be a better place to do this.