Why do we apply paint on iron articles?

Answer: When iron objects come into contact with moisture in the air, iron oxide develops and forms a reddish-brown layer which is known as rust. The chemical equation of this complex process is, 

4Fe+3O₂+6H₂O→4Fe(OH)₃ 

Therefore, paint is used to keep an iron article from coming into touch with moisture in the air. Paint protects the iron from rusting.