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I'm building a very simple work-from-home desk, with 3/4" AC fir plywood for the top. The plywood has a concave bow on the top side. How do I straighten this board?

Other details:

  • I've already applied several coats of polyurethane to the top--perhaps this was a noob mistake to do before straightening.
  • I'm planning to use sawhorses as legs, and screw a 2x4 lengthwise to the bottom of the desktop for rigidity.
  • Desktop is 72"x24".
  • I didn't finish the bottom. Do I need to do this for moisture reasons?
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    Please use the search, it works excellent here. There are at least two or three previous questions on flattening/straightening ply. – Graphus Jul 11 '20 at 9:57
  • "I didn't finish the bottom. Do I need to do this for moisture reasons?" With ply and other manmade boards, perhaps, not for moisture reasons exactly but because of the stress induced by the drying finish. This is similar to why counter-veneering is done, a basic veneer is applied to the other side of a panel to counteract the forces from the show veneer's glue drying. – Graphus Jul 11 '20 at 10:02
  • With solid-wood tabletops by the way there's no need to finish the underside, although many sources claim it's desirable or necessary most tabletops historically were not finished underneath, which tells us something fairly significant. Tabletops are held flat because of how they are installed, and when they bow it's not due to a weakness in the finishing regime but a flaw in the installation. – Graphus Jul 11 '20 at 10:03
  • @Graphus Thanks for the tip. Maybe I'm just searching with the wrong terms. I just tried "how to straighten plywood," and this question was the only result. With simply "straighten plywood" I get one additional result regarding rabbet/dado work. What is a better search phrase? – zwiebelspaetzle Jul 11 '20 at 16:12
  • I added the warp tag. Click on it to find additional related questions. – Ashlar♦ Jul 12 '20 at 2:22
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Plywood is difficult to straighten; it is full of glued together plys.

  • You could glue/screw another warped piece with opposing curves to it.
  • Screw/glue straight runners to the underside. 2x4's, angle iron, etc.
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