Question

what is rusting and why is paint applied over an iron metal

Answer 1

Paint prevents the oxygen molecules from interacting with the underlying material to form an oxide. It's a protective barrier. Rust is a common name for iron oxide, but most metals can oxidize.

Answer 2

Rusting is a chemical reaction which occurs in the presence of water and air. It is an irreversible change.
Paint is applied over a iron metal so that rust could not occur on the metal. Rust makes the metal dull.