In order to paint a thin section of wood with acrylics, you need to paint both sides at the same time. This will eliniate the warping since the tension on the wood that causes the warping is the sme on both sides, canceling out the warping. You need to hold that wood with a clothes pin attached to a board, so that both sides can be accessed at the same time. Once the paint is dry, you need go back and paint the portion of the wood that had been previosly covered by the clothes pin.
Tom