Un-warping plywood technique?

I cut a piece of 3/4" birch plywood about 10"x60". It will be the side of a book case. If I lay it on a flat surface it has a slight hump in the middle of about 3/16". Has anyone ever tried to undo this warp?
I have the board suspended between two saw horses with about 50 lbs of weight on the middle. the weight flexes the board plenty, but when I remove the weight it springs back. I left the weight on for 24 hours. I also tried warming the wood with a heat gun. Anyone have any ideas? do i have to leave the weight on for several days?
Note: This is the last time I buy ply at Lowes.
Thanks
Mitch
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Imagine the time it took to get that warp just right. You're not going to take it out in a day, maybe a week, put you'll probably need a few months.
A couple ideas: 1. Screw it in, as is. Sufficiently supported with screws, the warp will effectively disappear. (I think this is the best option, but you might be going for a different look from the back.) 2. Steam it to make it more receptive to being unwarped. Search on "steaming wood" or "steam bending" for more information. 3. Let it sit, flat, for months. It might fix it, it might not. Do this in an unheated building so your board will be subjected to more extremes in humidity and temperature. (Why your board warped in the first place, that is: humidity and temperature changes.)
Puckdropper
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First, most any plywood will warp to some degree. 3/16" isn't much warp, and should straighten out easily when it's applied to wherever it's going to be used. IMO you're very unlikely to be able to ever get it perfectly flat. It's more apt to end up warped in the opposite direction when it does change, if ever. If for some reason it's only going to be fastened on two sides or in some way that won't pull the warp out naturally, then you're probably best to figure some way to add runners to it to pull the warp out. Although I can't imagine any place to use a piece of wood that size where it wouldn't be well supported anyway, which normally pulls out the warp when it's attached. It's just the nature of plywood to warp unless you spend the extra money for the best grades. And how you stored it, of course, dictates a lot of the warpage, too.
HTH
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