With its purpose to lock out moisture and reduce corrosion, it acts as a barrier and protects against general damage or weathering. Most can be used on interior or exterior metal, without the need for a primer or undercoat, although it will give you a smoother finish. Ideal for metal furniture, gates and even as a garage door paint.  For outdoor projects, oil-based metal paints are the best bet. Oil paint is very durable, and is generally easier to clean.