A thick and strong door is one of the main things that keeps moisture out of gun safes. Most gun safes are built with at least four layers of security: a sturdy, heavy metal door, a sturdy metal frame with deadbolts, a lock system, and maybe a combination lock. The door should also be weatherproof since the only thing that keeps out moisture are the hinges and seals around the door itself. But a well-built gun safe will also have a heavy-duty weather seal on the back and sides of the door. You should also ensure that the safes have good insulation and a fan system to remove excess humidity. These measures will help you to avoid mold, mildew, and any damage to the interior of your safe.