It also depends on what the mask is for. If it is a decorative mask to be hung on the wall and not worn, then you could use a paste that has poison in it such as some varieties of vinyl wall paste. If the mask is to be worn, you can also use various coatings on the mask, such as water-based varathane, found at http://www.polyclay.com/fletco.htm Or sculptorcoat. If you store your masks in a trunk, Bay leaves, catnip and basil kept in cheese cloth bags will usually keep most critters away.
James