An iron article should be coated with paint to prevent rusting.

Iron articles when left undisturbed, often undergo rusting resulting. As a result of which, a reddish-brown layer gets formed on its surface. 

It is due to the fact that upon exposure of iron to the air and water in the surroundings, they do a chemical reaction. By painting an iron article, the paint prevents the interaction of iron with the surrounding air and water. In other words, It protects iron from rusting.