warped plywood

a few months ago i bought 10 sheets of 1/2" okume plywood for an interior cabinet making project.
I stacked them on top of another sheet of 3/4" that was on chocks (two at the sides and one in the middle) and leveled, and they were stored inside my workshop.
Actually that 3/4" sheet was the last of 10 sheets I had stored in the same place for over 1 year, and they were all straight.
I live in the tropics (Costa Rica), but it has been dry season since i have the sheets, so humidity has been fairly low and constant.Actually I would say that the place were they were stored has been pretty hot.
Now that i'm ready to use the 1/2" sheets, i found that they are warped (all except one that was in the middle of the stack), the 3/4" old sheet at the bottom is still straight.
Now the strange (to me) point is that the stack looked straight, actually it showed a little 1/16 concave bow in the middle; but as i started lifting the first sheet, it was warped in a convex shape of 3", and as i went trough the whole stack i had the same result, except for one in the middle that was fairly straight.

Any idea on what happened??

Any suggestion on how to straighten them back?

 

put them on the ground separately...convex side up. As the outer layers absorb the sun they will dry and contract and the bottom will stay cool and absorb a bit of moisture from the ground...straightening the sheet. Watch them as you can overdo it and get the opposite bow. Problems of mass production...stresses in the veneers... moisture buildup between the sheets...who knows. I do know the sun trick works...and rather quickly too. Next time try stickering them with some 1/4" slats between the sheets...enough to keep them flat and let air circulate all the way around. I have a stack in my soft garage that will probably do the same thing even though I stacked them on foam insulation to separate them from cooler temps near the (tarp covered) ground.

 

If straightness is permanently compromised, you may be able to use them if you apply epoxy and 6 oz fg to one side when the sheet is held perfectly flat on a thick sheet of mdf ( usually with another heavy sheet of waxed mdf on top ).

If you apply peel ply to the topside of the fg, you will end up with a nice smooth surface, and a permanently flat bit of ply.

Usually a thin layer of fg and epoxy will not be a problem for most projects, whereas bowed sheet is a real pain.

 

Steve has it right and this is an old trick. You don't even need the sun really, you can use a spray bottle of water, spraying the side you want to curl towards. Try it; take a sheet of relatively thin plywood, stand it on edge and spray one side with water, then let it dry.

 

I will try to use the spray trick, so if i'm not wrong, i should spray the concave side and then put it face down (convex up) on the floor and wait for moisture and gravity to do the rest ...i will avoid the sun, first because i don't have a leveld surfece in sun light, second because i'm in the tropics and sun here is really powerfull!!
I'll let you know of the result .....

 

Its worth a try for sure - but once bowed, I have never managed to get ply back to its proper flat shape entirely.

Once the cells have dried out, they don't seem to take up the moisture evenly any more.

Good luck though

 

I've had pretty good luck with straightening plywood that was just bowed -- at least, long enough to get it nailed down. But I've never been able to straighten plywood sheets that are twisted, so that two opposing corners touch the floor and the other two stick up in the air.

When I get one like that (and they're pretty common at big-box lumber stores), I use enough fasteners to force it flat anyway, if I can. Otherwise I cut it up and use it where I need smaller pieces.

 

Wouldn't ya know...I decided to practice cranial-rectal insertion and leave a sheet of ply out today and it rained just enough to turn it into a curly-Q...I'll let tonight's dew straighten it out with the help of a couple of old batteries. I have others but this one was the best of the bunch. I hope I can get it straight enough to be useful to me.

 

Boiling water soaked rags and/or steam can be helpful too, then clamp flat to dry.

 

I figure a sheet of marine play between $60 and $120 is worth spending $15 of 6 oz cloth and epoxy to get back to its precise shape - especially when in 99% of the time, its gotta be epoxied or painted anyway.

 

Most plywood has to be bent into place anyway, plus cut to fit, so it's not as big a deal as you'd think. Of course a bulkhead would look a lot better if it was true, but you could brace it into place and tab it in easy enough.

 

Actually...this is for the center bottom panel of a canoe and it bent the wrong way...I'll just have to un-bend it and get it going the right way. There is only 3" of rocker over 7'+ fore and aft of midships so I won't need much.

 

Well...here is the ply after having left it overnight 'tother way around and then baking it today. Bear in mind that the ends of this thing were curling a good 18" off the ground last night...

 

That's a big improvement for sure - very few kinks left.

I guess the big test is to drape it over a couple of frames imitating the curve of the canoe bottom, and see if you are getting any 'ripples' showing up. A torchlight at dusk seems to show any problems up best.

These ripples often don't show up till you are at the painting stage (after the fg/epoxy is laid on) , and show up as big curves or dips that require a lot of filler and sanding.

This is the bit where I laid thin plastic over a thick melamine covered board, put the plywood on it, applied 6 oz/cloth and epoxy to one side of the plywood under peelply, and laid heavy, plastic covered sheets of flat particle board on top to press it down flat.

The result was a pre-glassed, perfectly flat panel that had zero ripples in it.

Might be a bit of overkill for your situation, but it works if you want the best finish for the least work.