definitely plastic...we spray the backs to let their wrinkles relax... you don't want to worry about the dampness...
tip...i put my zippers on differently than their suggestions...if you put both the female sides on with the zipper handle at the right side and both mail sides to correspond to that, then you can take the quilt off and turn it 180 and put back on the other direction.... this helps a lot when doing borders intricately...