Primer allows the paint to adhere better by providing a uniform surface, and will help it last longer. It can also seal a porous surface, block out stains from bleeding through the topcoat, prevent rust formation on iron and steel, and prevent tannin bleed on wood. ... For painted wood, no primer is needed.

What happens if you paint metal without primer?

Without the use of a primer, in many cases, oxidation will begin leading to rust and the eventual decay of the metal. In addition, the chemical makeup of paint is less binding than primer meaning it will flake off easier without a primer underneath it.

How do you prepare metal for rustoleum?