Outdoor signs have to be able to withstand the weather and UV rays from the sun cause the most damage. They fade colors and reduce the lifespan of paints. This is particularly damaging to wooden signs because when the paint fails, water permeating the sign spawns wood rot and insect destruction begins. Plastics are designed for outdoor use and have greater longevity. Powder coated finishes or baked enamel paint finishes for metal signs do better yet. It is important that all outdoor signs are designed in accordance with their intended use. A site sign, for instance, typically needing a limited life span can safely be made of MDO board (plywood). More permanent signs demand other substrates.