Garage storage cabinets need not have mildew buildup if you follow some common sense advice. The first piece of advice is to always clean and oil any tools that you may be storing in the unit. Likewise, a coating of light machine oil on the walls, doors and shelving will prevent mildew buildup, as will good ventilation. For ventilation, consider using a small ventilation fan in the top and air holes in the bottom. It also never hurts to ensure that there is a good vapor barrier underneath the storage to keep condensation, mold and mildew out of the cabinet. If you do experience any mildew, use a mild solution of bleach and water to clean it up.