I’ve got an old game that has a piece of cardboard (about 12" x 17") that attaches to the back of a plastic frame. It has slight curl to it that prevents it from sitting tightly in place. Any tips on flattening this cardboard piece without wrecking it? It has been lying pressed under weight for a couple of days but that hasn’t helped very much.
Just a guess but you might try steaming it and then pressing it flat to dry.
Try applying a counter-curve by laying the piece so that the convex side faces up (so that it is resting just on the edges). Next, support the edges with soup cans or what-have-you that will provide good clearance off the table or whatever it’s sitting on. Next, apply gentle force with a weight just sufficient to push the convex side barely concave. Let it sit like that for a day or two and see what happens.
The only way to straighten cardboard it to add moisture and then press it flat until it’s dry. I would use a steam iron to steam the back side, and then weight it down until dry. The less water you add to the paper the better. you don’t want to saturate it too much.