Ellis
.. or exactly what you mean by that, but the foolproof way to flatten or otherwise counteract distortion is with structural members that have enough engineering in them to overcome the problem. Sometimes, this even means crowning the ledgers or frames or whatever.

I wouldn't bother with the water treatment. Clamping flat won't help in any case. Over-clamping might work temporarily, but I really think you need to engineer a structural solution.

Ellis