I am new to woodworking, and I have a lot to learn about plywood. When I got a table saw last year, I needed some inexpensive wood to practice, build jigs, etc. so I bought the cheapest wood I could find: 3 white SPF (softwood) boards, a poplar (hardwood) board, and one sheet of the cheapest 1/2 inch plywood they had. All I wanted was a flat sheet that I could use for projects and practice.
I had remembered plywood from the 1970s. I expected gaps, knotholes, and other imperfections, but I expected flat
. I ordered the plywood at the lumber desk, and I specified two cuts so that it would fit in my small-ish SUV. Next, I drove around to pick up the plywood, and was stunned at how warped and twisted it was. I felt like they had chosen the very worst board for me, but it was already cut, so I took it home. The warp was many inches between corners, much worse than reported above.
I took one of the cutoffs and tried to flatten it as a test. I wetted the concave side with the hose, placed it face down on the concrete garage floor, and stacked paving stones on it to weigh it down and flatten it. I left it for two days, but had no luck. There was no change in the board.
What worked was time. Not knowing what else to do, I placed the plywood with my other boards. They are vertical, touching the concrete on the bottom; they are bungy-strapped to a folded ping-pong table. Over time, the plywood flattened itself reasonably well, just standing up, sandwiched between other boards. It was never perfectly flat, but it was flat enough to build things with it. I am using it for shop projects, such as the cyclone/shop vac cart I made from it:
The rest will get used for shelf supports in the garage:
Don't think that just because you buy expensive, premium-grade plywood that it will be (or stay) flat. I bought a piece of 1/4 inch genuine Baltic Birch to become drawer bottoms, and it warped in that same ping-pong table storage that I am currently using. How you store your wood counts, too. I am planning to put the boards on shelves with wood stickers (for airflow underneath the boards) in the near future, and make a rolling storage cart for the flat stock, such as plywood. Hopefully the warped pieces will straighten themselves out over time.