Fixing warped masonite? Then painting it?

My dollhouse is mostly masonite which has warped, anyone have any tips for flattening them out again? So far I've seen suggestions of getting them wet and putting a weight on them. Any idea how long I would need to weight them down?
One side of the boards is painted with a wipeable surface "paint" but I'm hoping to repaint them if that's possible.
One side of the boards is painted with a wipeable surface "paint" but I'm hoping to repaint them if that's possible.