Purchasing plywood for your next building, remodelling or crafting project can be an overwhelming prospect, so once you settle on the proper size and thickness of your boards and purchase the wood, you'll likely feel a sense of relief. However, plywood left out in the sun or exposed to too much moisture can warp and change, leaving you with nothing more than an unusable mess. With careful application of weight, however, you can gradually flatten your plywood again.
Set the plywood on a flat surface in front of you with the concave side (the side that is bent in) facing you.
Fill a spray bottle with hot water from your sink. Spray the concave side of the plywood thoroughly with the hot water, until the side facing you is thoroughly moist but the water does not absorb more than halfway through the plank.
Set the plywood plank concave-side down on a walkway or driveway in the sun. If you cannot place the boards outside due to weather conditions, set it on the floor in a dry, warm area of your home. The dry air and heat (either from the air or the sun) will dry out the bumped-out side while the moisture absorbs into the concave side. This will flatten the plank.
Check on the warped plank every 30 to 45 minutes until the plank is flat again. When you have straightened the plank, store it on its side in a cooler area of your home until the plank dries completely; this should prevent further warping.
If you find that the plywood plank warps the other direction as a result of your work, simply apply a little moisture to the newly-concave side and repeat the procedure. This should not happen as long as you check on the plank regularly and store it properly once it is straightened out. You can also straighten plywood by taking two warped pieces and gluing them together, then applying strong pressure to the warped areas with a heavy object. Remember that this will change the thickness of the plywood, since you have to glue two pieces together.