The steel signs are painted with 3 coats of Rust-Oleum enamel paint front and back and although the paint will last a very long time, it will not last forever. Here's a few things you can do to make it last even longer. 

• Keep the sign out of direct sunlight and weather as much as possible 
• A good car wax like a carnuba wax once or twice a year will help protect it from the elements
• Wash it with mild soap and water once a year and reapply wax
• Repair any nicks or deep scratches
• Repaint it every 5-10 years