How to avoid bad plywood
I have a number of plywood sheets in my shop—some partial, some whole—that have warped, making them difficult to work with. How can I flatten them?
—Leroy Zahm, Laramie, Wyo.
You'll find it suggested far and wide, Leroy, that you can flatten plywood by restoring the moisture equilibrium between the top and bottom plies. That entails adding moisture to the concave side, removing it from the convex side, or both. One popular notion suggests doing both by laying the sheet on your lawn on a sunny day. We've tried those ideas and have found the results unpredictable, at best. Rarely does the sheet return to a flattened state. Our advice: Avoid warped plywood altogether by buying high-quality material, using the material before it has a chance to warp, and storing it correctly.
When shopping, stay away from bargain-basement plywood. It will often warp soon after you pull it from the stack of other sheets holding it down flat. If available, choose a combi-core product that has a plywood center, with MDF or particleboard layers on both sides of the center, and veneer skins. In our shop it tends to stay flatter longer than other plywoods. Products with cores of only MDF or particleboard also tend not to warp quickly, but weigh a lot and don't hold screws well.
After deciding on a plywood, buy it the same day you plan to cut it into project parts. Unlike solid wood that needs days to acclimate to your shop's environment, plywood should be cut up as soon as you get it home.
If you don't use all of the parts that day, clamp them together, sandwich-style, with other materials that tend to stay flat such as particleboard and MDF. Do the same with leftovers. Clamped this way, you can even store the pieces standing up to save space.