"Exit Only - Do Not Enter" Sign By SmartSign | 12" x 18" 3M High Intensity Grade Reflective Aluminum 3M AUTHORIZED. Signs use 3M reflective High Intensity Grade film mounted to 63 mil thick aluminum. This assures that signs are durable outside. DURABILITY. 10-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. LAMINATED. Graphics are protected from weather and abuse. Graffiti can be cleaned off. Laminated signs outlast competitive unlaminated or overcoated signs. INSTALLATION. All 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.