Warped gessoed art-board


Code Dragon Slayer
Is there a trick to un-warp an art-board? It's the first time I use art-board, and though I made a X on the backside with gesso like I read, the art-board warped anyway, I added an other layer of gesso on the back, but it hasn't done anything.

Now maybe I went about it in the wrong order, maybe I should of made the X and let it dry before gessoing the front (I gessoed the front then immediately made the X in the back).

Is it too late?

Thanks for any help you can give. :)
probable have to paint the entire back of the board,, if you put a thick coat on the front