"15 Minute Parking" Sign by SmartSign | 12" x 18" 3M Engineer Grade Reflective Aluminum 3M AUTHORIZED. Signs use 3M reflective Engineer Grade film mounted to 63 mil thick aluminum. This assures that signs are durable outside. DURABILITY. 7 year durability backed by a 3M warranty and rustproof aluminum. ROUNDED CORNERS. Signs have rounded corners and burr-free corners for safe handling, longer life and a professional appearance. INSTALLATION. Most signs come with pre-punched and pre-cleared mounting holes for easy installation. Signs mount to U-channel posts, square or round galvanized posts, wooden posts and chain link fences.