It helps a lot to start out with warp free wing parts. Check your leading and trailing edges as well as your struts. If they are warped, either replace or get the warp out before you install. A bad trailing edge will easily flick some washout or lift into wing no matter how straight a surface you start out with. A warp in the struts can be dealt with and the instructions will tell you how. I use the Edwin Lind Magnetic Board and it is great but it alone will not prevent a warp.