Panels that have bowed need to be brought back to flat again. A little bowing/ warping will not matter so much if the panel is to be fastened inside of a frame. However, try the following to see if you can make the panel flat. 

To counter-bend the panel, you’ll need three bricks - or some heavy, similar-sized objects. Spray the back of the panel with water. Get it wet. Position two corners of the panel on the bricks. Place the third brick in the middle of the panel to counter-bend the panel. Leave it overnight. This should improve the bowing/warping. Once everything is flat then you can coat the back with paint or medium to maintain the re-shaping.