Typical resins which can be used as the protective layer are cellulose derivatives such as ethyl cellulose, nitrocellulose or cellulose acetate; acrylic resin such as polymethylmethacrylate, polyethyl methacrylate or polybutyl methacrylate; vinyl resin such as polyvinyl acetate, vinyl chloride-vinyl acetate copolymer, polyvinyl alcohol or polyvinyl butyral; and thermosetting resins such as polyest