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I posted here (Question on relative warp resistance of engineered wood ) to this forum and got help for a wood sheet based project.

My plywood sheet arrived about 5 days ago it is softwood and has a sort of veneer on either side.

There is an issue. There is a 2mm high (approx) raised area at one end, highest at the edge of the wood which comes down approx 30cm and is roughly 30cm in diameter if it was a circle. (I know it sounds pedantic, but the wood is for a person with a severe spinal condition )

My guess is that the hump/raised area came about as a result of a clamp holding the wood in position, perhaps during storage and transport or cutting itself. The raised area/hump is at the rough centre of the edge.

rough diagram showing the raised area

I have not examined underneath the bed yet to see if the raised area has the mirror image shape missing as is upthrust from the top yet, but that would be tricky to do.

My experiences with mdf have not been enormous but it seemed to be flat, for sure and to not have raised areas like described.

My question: - Are such variances (ie 2mm humps not uncommon) part and parcel of using plywood? - If they are common am I right in guessing that mdf is less likely to have a non-uniform surface?

Many thanks in advance gav

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  • Will the person be laying directly on the wood, or will there be any form of padding on top? A 2 mm bump isn't standard or expected for finished plywood because it is typically used for furniture. Can you return it as defective?
    – fixer1234
    Sep 9, 2018 at 23:58
  • thanks - lying on a mattress on top of the wood. Returning it as defective is not something I had thought of though. thanks
    – user89970
    Sep 10, 2018 at 19:01

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2mm humps are not common with good quality plywood. You could plane or (with effort) sand that flat without impacting strength. (Note that you would impact the finish, as you'd go through the veneer into the inner plies.

MDF would be flatter, but not as strong as plywood. If you need dead flat and a nice finish, (and you had room) you could consider putting a thin skin of mdf (like, 6-12mm) on top of the ply that you'd flattened. (You might even get away with not flattening the ply, as the mdf skin would make the 2mm difference very hard to perceive.)

Alternately, and you'll have to judge if this is appropriate for your application, you could put the hump at the bottom of the bed where it might be less noticeable.

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  • thanks for that. I examined the underside now and can see the hump has come through from the other side. For my purposes, I need to flat surface and a divot is still not a workable solution. But thanks for the pointers.
    – user89970
    Sep 10, 2018 at 19:04

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