Gator Board has a very rigid poly surface which is resistant to moisture, unlike foam board which is foam covered with lightweight paper stock. It is an extruded polystyrene foam encased between layers of a melamine and wood fiber veneer. Gatorfoam / Gator Board has a much harder outer surface than other foam core boards and resists warping. It is not an acid-free material, so you should check before using it for items which will be displayed in museums or other permanent archives.