"Enter Only" Sign By SmartSign | 18" x 18" 3M Engineer Grade Reflective Aluminum 3M AUTHORIZED. Signs use 3M reflective Engineer Grade film mounted to 80 mil thick aluminum. This assures that signs are durable outside. DURABILITY. 7-year durability backed by a 3M warranty and rustproof aluminum. 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. LAMINATED. Graphics are protected from weather and abuse. Graffiti can be cleaned off. Laminated signs outlast competitive unlaminated or overcoated signs. ROUNDED CORNERS. Signs have rounded corners and burr-free corners for safe handling, longer life and a professional appearance.