Mathematician's Merrimac
Cupped Plywood
Try this, Put your cupped plywood concave side down, out in your yard, on the grass on a sunny day, Let it sit a while. The moisture from the grass makes the down side of the ply expand while the hot sun dries the top side making it contract. Take the plywood into the shop and set it flat with weight on it to keep it flat. When it dries it should remain flat. Doesn't always work but it is worth a shot just keep an eye on the ply when it is cooking don't want it to cup the other way.
Try this, Put your cupped plywood concave side down, out in your yard, on the grass on a sunny day, Let it sit a while. The moisture from the grass makes the down side of the ply expand while the hot sun dries the top side making it contract. Take the plywood into the shop and set it flat with weight on it to keep it flat. When it dries it should remain flat. Doesn't always work but it is worth a shot just keep an eye on the ply when it is cooking don't want it to cup the other way.