Unless it is galvanized, bare metal will rust if it's exposed to moisture and oxygen. Once rust sets in, it will ultimately destroy metallic surfaces unless it is sealed with the appropriate rust-inhibiting primer.
Never apply paint to unprimed rusted metal, or the rust will bleed through. Take the time to prepare the metal for the primer, or poor adhesion will lead to finish failure.
The Wrong Primers
Ordinary acrylic, latex and oil primers will not permanently seal rust. If you apply these types of primers on top of existing rust, you'll see a noticeable recurrence within months.
In addition, these types of primer typically bond poorly to metal and aren't appropriate even when no rust exists.
Iron Oxide Primer
Iron oxide primer is formulated to seal existing rust and prevent future outbreaks. This particular primer has an oil base and a reddish color.
Apply iron oxide primer with a natural-bristle paintbrush. Don't use synthetic brushes, or the iron oxide will ruin the bristles.
Zinc Chromate Primer
Zinc chromate will also seal rust and prevent a recurrence; however, it is highly toxic. For this reason, zinc chromate primer is difficult to find.
Although highly effective at sealing rusted metal, this particular primer isn't well-suited for use by inexperienced do-it-yourselfers.
Rust-inhibiting Metal-etching Primer
Rust-inhibiting metal-etching primers not only seal rust and prevent it from recurring, they are formulated with an acidic base that allows the primer to etch slick metallic surfaces. This drastically helps to improve surface adhesion.
Use a rust-inhibiting metal-etching primer on very slick metallic surfaces affected by oxidation.
No type of primer will bond to loose, flaking bits of rust. Before applying any sort of rust-inhibiting primer to metal, scrape away loose rust, using a wire brush and coarse, 40-grit sandpaper.
Wear gloves during the removal process to prevent possible injuries.