Painting gesso on the back will cause the panel to warp away from the gesso, or towards the front, so don't do it if it is warping that way already. Apply it to the side that is cupped not bowed. It will only get worse if you apply gesso to the bowed side. Also, once it fully dries any warping "adjustment" done with gesso can often go away. If it is warped away from the back, t is probably warped because the back was not sealed.
Try laying heavy stuff on top of it on a flat surface for a month.
Originally Posted by theBongolian
if I'm looking this up correctly dibond costs TEN times + as much as hardboard (masonite).
If you get 3/16" masonite instead of 1/8" masonite, you will have virtually no warping. I use both 1/8 and 3/16 and don't have any signifcant warping.
I use both tempered hardboard and ACM panels. I don't buy Dibond brand as it is too expensive plus the metal facing is thinner than some less expensive brands.
If money were no object I would never bother with hardboard, but the cost savings is significant.
4x8 Sheet of 1/8 tempered hardboard from Lowes $8.
4x8 Sheet of 3mm Montroy M-Panel $42
Not exactly ten times the cost, but it is over 5 times more. The ACM panels are MUCH easier to cut than hardboard and are a significantly better support for all kinds of work. Another nice thing is since you don't need to use a saw to cut them there is no loss from a saw blade with the ACM. That means you can make 4 cuts at 12" wide on a 48" width, which you cannot do with hardboard since the saw blade eats up about 1/8th of an inch per cut.