"Emergency Exit Only - Door Must Remain Closed at All Times" Sign by SmartSign | 10" x 7" 3M Reflective Aluminum 3M AUTHORIZED. Authorized signs are certified to the 3M Matched Component System. This assures that signs are durable outside. DURABLE ALUMINUM. Signs use 3M reflective Engineer Grade film mounted to 40 mil thick aluminum. All signs come with pre-punched and pre-cleared holes. Signs last up to 7 years outside. ROUNDED CORNERS. Signs have rounded corners and burr-free corners for safe handling, longer life and a professional appearance. INSTALLATION. With our signs, installation is a breeze. Most signs come with mounting holes for easy installation to chain link fences, walls, and doors.