you might attach a piece of 1/4″ plywood to the back – making what’s called a “mirror” back. So if you had a frame that had an outside dimention of say 20 x 24 – you would first put the painting in the frame then screw a piece of 20×24″ ply across the back of the frame. it depends on the type of frame and the amount of warping. but this can work in some cases.