How do I paint rusty metal so the rust does not come back?

Iron and steel rust because iron, oxygen and water react, and the rust that forms is porous and holds more water against the metal. That is why rust creeps sideways under a coating: the paint keeps rain off the surface but the reaction underneath already has everything it needs. A paint job on rusty metal is only as good as the boundary you leave between sound metal and what is left of the corrosion.

Safety

What you'll need

• Wire brushes, and a wire cup or twisted-knot wheel on a drill or angle grinder
• Abrasive paper or an abrasive pad, and a degreaser
• A rust-inhibiting metal primer, or a direct-to-rust metal paint
• Rust converter, for pitted metal you cannot get fully clean
• Rust-penetrating oil for seized hinges and fittings

Step by step

1. Work out how much sound metal is left.Tap and probe. Surface rust on solid metal is a preparation job. Metal that flexes, flakes through, or has holes and heavy pitting through the section is a repair or replacement job, and painting it only hides the problem until it fails.
2. Remove everything loose, and then some.Wire brush, scrape and abrade until nothing more comes off and you are down to either bright metal or tight, adherent rust that cannot be dislodged. A wire cup brush in a drill does in minutes what a hand brush does in an hour. Feather the edges of any sound old paint you are leaving.
3. Decide between clean metal and a rust converter.Bright metal plus a rust-inhibiting primer is the strongest system and the right answer wherever you can reach. Where the metal is pitted and you genuinely cannot clean the bottom of the pits — ornate railings, box sections, corners — a rust converter, which reacts with iron oxide to form a stable black compound, is the practical alternative. Follow its instructions exactly, including the overcoating window.
4. Degrease and prime the same day.Bright steel starts to flash-rust within hours in damp air, and any hand grease or old wax kills adhesion. Wipe down with a degreaser, let it flash off, and get primer on while the surface is clean and dry. This is the step most amateur jobs lose.
5. Choose the coating system for the exposure.Zinc-phosphate or zinc-rich rust-inhibiting primer plus a metal topcoat is the traditional and reliable route. Direct-to-rust or direct-to-metal single-tin paints are genuinely convenient and work well on tight rust, but read what they require underneath — several still want their own primer over bare or heavily corroded metal.
6. Build thickness where water sits.Corrosion always starts at the bottom of a post, at the base of railings where they meet stone, inside box sections, and at every joint and weld. Give those places an extra coat. The face of the metal is not what fails first.
7. Seal the places water gets in.Open top ends of hollow posts, drainage holes that have blocked, and gaps where two sections meet all admit water that cannot dry out. Cap, clear or seal them as part of the job — a perfect paint film outside does nothing about rust starting inside.

Tips

• The best time to paint outdoor metalwork in Britain is a dry, mild spell with no dew forecast overnight. Metal is colder than the air first thing and last thing, and paint applied onto dew never adheres.
• Cast iron rainwater goods, gates and railings repay a proper full-strip job once, followed by touching in every couple of years, far more than repeated coating over old failure.
• Rust converter is not a primer and not a paint. It stabilises rust and then has to be coated, usually within a stated window.

Common mistakes

• Painting over flaking rust to buy time — The rust reaction continues, expands and pushes the new film off from beneath. You get a year, and a worse surface to prepare next time.
• Leaving primed metal bare for weeks before topcoating — Most primers are not designed for long UV exposure and will chalk and lose adhesion. Follow the primer's overcoating window.
• Dry-sanding old paint on Victorian railings or metal windows — Pre-1990s coatings may contain lead, and dry-sanding turns it into a dust you and everyone nearby breathe. Use wet abrasion or a chemical stripper.

If it doesn't work

Keeping on top of it

• Walk exterior metalwork and touch in any chips, scratches or rust spots before they spread (Twice a year)
• Clear drainage holes and check the bases of posts and railings where they meet stone or concrete (Annually)
• Wash salt and grime off coastal or roadside metalwork (Annually, or more on the coast)

Questions people ask