I am trying to make table tops from 1/2" Baltic birch plywood. It seems flat when I get it from the lumber yard. After painting (both sides) my painter stands each piece on end to dry. Drying might take several days. Most of the dry painted pieces have a slight warp afterwards. My painter tells me that if I stack them flat for a period of time, they might return to flat. Is it the painting that warps them? Or the vertical standing that allows them to warp? Will it come out if stacked flat?
There's really no way to end up with flat panels, even from the main stack when you buy them, they typically aren't flat.... especially 1/2". Wetting the surface with paint and then having each side dry independently is causing it to warp a little more...but the way the plywood is made is also responsible..the plies are arranged perpendicularly to each other, and each ply exerts a little tension on the panel...if you haven't designed some sort of frame to hold the panels flat, you won't be able to produce a flat surface, a thicker plywood will resist this more, but, again, typically there needs to be some frame or structure...plywoods also differ in quality, (and number of plies, species, etc), and cheaper plywoods with less plies or of a softer or inferior species, will be more prone to not being flat. A Baltic Birch or similar panel, has many plies, and the quality control and controls in the factory are such that they can produce a much higher quality and more stable panel...albeit at a substantially higher cost as well.(the old "you get what you pay for" adage)....there are many qualities and kinds of panel goods available, I would stay away from the Home centers and such and find a plywood retailer...but even with that a 1/2" sheet that will be and stay perfectly flat is not a reality...you would likely be better off with a panel of MDF...but still, one needs to follow established construction techniques to allow for a flat surface, meaning a frame, apron, or some type of substructure to hold the panels flat.