Plywood movement

I cut up some 3/4 ply for a small bookcase. Unfortunately, some of the pieces have moved (cupped). This was NOT cheap HD stuff. Went to the cabinet supply warehouse and got 7 ply stuff. About $65 a sheet.
Any hints on "decupping"? I clamped the pieces together (four shelves) with the cup facing each other and I've noticed some improvement.
Is this that common? I live in Northern CA, where it's been dry since May. Not to say we might have had some humidity. My neck of the woods gets a lot of overnight fog.
MJ
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On 8/28/2012 7:04 PM, MJ wrote:

Cheaply made, not cheap to buy, plywood will absolutely cup if exposed to sun direct light. If this was the possible cause, try exposing the opposite side.
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On Tuesday, August 28, 2012 5:04:39 PM UTC-7, MJ wrote:

No telling exactly what caused it but not very likely any of the ol' tricks will really fix it.
Old tricks: - Lay on grass cup down during sunny day so moisture enters cupped face and expands\straightens. - Clamping in some flattening configuration. I would suggest that adding a hardwood edge is liklely the only way you can pull it into compliance. Use a spline along the edge to really lock it in.
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I find that sheet goods inevitably cup/warp after I buy them. If you look at the piles in HD or Lowe's in the evening the top sheets of many piles are not flat regardless of the specific item and even on freshly unbanded units. It is due to one side of the sheet being dryer than the other. I try to buy sheet goods a couple weeks before I need them. Then, when I bring new sheet goods into the shop, I leave them on edge against a straight surface with an air space between the sheets (think stickers). While the sheets may go wild the first few days after a week or two the sheets tend to straighten themselves out with no assistance from me, i.e., they normalize. There have been exceptions, e.g., lumber core 3/4" luan ply, but they are exceptions. The alternative is to cut the stuff up and assemble quickly, or cut it up and stack with weight on top and then assemble quickly at a later time. Neither of which EVER happens in my shop... it's a hobby, not a job, so I don't move quick enough!
Given that you have warped parts you might try stickering the pieces like solid wood and weighting the top of the pile. In a week or two you will likely find that the pieces have dried evenly and normalized. It's a conservative approach but it seems to work pretty well in my experience.
John
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On Tuesday, August 28, 2012 5:04:39 PM UTC-7, MJ wrote:

Just to be clear, this is about 0.750 inch thick plywood...

Again, to be clear, this is not Unfortunate, this is Predictable, and ALL of the pieces have moved. Some were cupped.

All wood responds to moisture changes, and no amount of money can get you real natural wood that doesn't.

The reason wood moves, is differences in moisture uptake from the air. Putting the concave (i.e. dry) faces together will help if and only if the weather is dry and the moist faces (which are exposed) are drying and shrinking. The usual recommendation isn't stacking, it's VENTILATED stacking.
Probably, though, you can just assemble your parts and ignore small curves. If the boards spontaneously straighten after assembly (it's likely) you can then apply a finish to all surfaces, and slow the movement in future. The plywood was flat and at uniform moisture content once (at the factory) and will return to flat if/when its moisture content is again uniform.
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