Things You'll Need
Books or bottles of water
Small plastic tub
Larger plastic tub with lid
The same thing that warps cardboard -- humidity -- can sometimes help you straighten it out again. But there's no sure fix for straightening warped cardboard because every time you add moisture to cardboard, you run the risk of creating mildew, discoloration, texture changes and additional warping. The best solution is to prevent warping in the first place, but if it's happened and you can't live with it, you can try one or all of several tricks to straighten it.
Lay the cardboard out on a piece of plywood or a flat, solid wood table, with the largest convexity or convexities -- where the cardboard is warped to bulge outward -- facing up.
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Place a sheet of plywood over the cardboard and weigh it down with books or bottles of water.
Leave the plywood and weights in place for two to three days; then remove and check the shape of the cardboard beneath. Because this method doesn't involve any moisture, you can replace the weights and leave the cardboard in your makeshift press for as long as you like; over a period of days, the warping may gradually ease.
Cut two pieces of plywood to fit the bottom of a plastic tub that's large enough to hold your cardboard flat in the bottom.
Place one piece of plywood in the bottom of the plastic tub, and place the warped piece of cardboard on top of it with the largest convexities facing up.
Add an inch or two of water to the bottom of a second, lidded tub that is both larger and taller than the first plastic tub.
Place the small plastic tub, with the cardboard inside it, into the larger plastic tub. Place the lid on the large plastic tub.
Lift the lid and check on the cardboard at least every 12 hours for two or three days. Remove it as soon as you notice the warped fibers starting to relax. Do not leave the cardboard in the tub long enough for it to become saturated with moisture, which will ruin it.
Remove the plywood and the cardboard from the smaller of the two tubs. Place the second piece of plywood atop the first, sandwiching the cardboard between them, and place books or bottles of water atop the second piece of plywood to weight it down.
Check on the cardboard every 6 to 12 hours to see if it's dried in place.
Keep your stash of multimedia art supplies or card stock from warping by storing it in a dry, well-ventilated place with constant humidity and temperature.
Any time you combine moisture with cardboard or card stock, you run the risk of creating a hospitable environment for mildew. Anything you can do to increase airflow, including pointing fans at the cardboard as it dries, will help prevent mildew -- but ultimately, living with the warped cardboard is the only no-risk proposition.
Do not let water touch the cardboard directly, and do not let the fibers become saturated with moisture; both will ultimately damage and warp the cardboard further.