I only ask because PVA glue can cause some warping on cardboard, and I'm actually finding some warping on foam core in one of my current projects. It seems to relax if I set something heavy on it overnight to hold it flat. I assume that the issue is uneven drying times so you get some expansion when the material is wet and then PVA tends to shrink and pull as it dries. Not an issue on something like MDF, in my experience. Not even an issue in really thin card or paper. I have mostly figured out how to work with it, but it is something I'm conscious of when working with PVA glue.