One of the most popular substrate choices for smaller (less than 48”) signs, aluminum is slightly more expensive than HDO Plywood. While less susceptible to porcupine damage than plywood, it is more easily damaged by bullets and more likely to be stolen by people wishing to recycle the aluminum. Expansion and contraction of aluminum may contribute to early sign failure.
High Density Overlay Plywood is a specialty plywood made to withstand extreme weather
conditions. The most widely used substrate for Forest Service reflective signs, HDO provides a stable surface for routing, is more easily repaired than some alternatives, readily available, cost-effective, and less subject to vandalism. However, it is often eaten by porcupines.
Medium Density Overlay Plywood is usually routed and painted to produce a lower cost option to redwood or HDO plywood. It is economical, easily maintained, and practical for large routed signs.
Available for smaller-size signs, Altree is a composite of wood chips and recycled plastic. It can be either routed and painted or used with reflective sheeting. Environmentally-sensitive, Altree is animal-resistant, very durable in a wide variety of weather conditions, easy to repair, and cost-competitive with HDO plywood.
Although long-lasting and handsome, routed redwood signs are very expensive and almost always crack with age. They will soon be phased out due to the lack of availability of clearheart grain. These durable signs can be stained or painted. Oak signs are normally reserved for wilderness areas.