Railings live or die on the prep, mate. The paint is the easy bit — it's the rust and the dodgy old flaking layers that catch people out.
Start by knocking off all the loose stuff. Wire brush, scraper, and a sanding disc on the really crusty bits. You want to get back to sound metal or sound paint — anything flaking will take your new coat with it. Wipe down with a degreaser or sugar soap, then let it dry properly.
Now the critical step: bare metal must be primed with a dedicated anti-corrosive metal primer — a zinc-phosphate primer is the classic. This isn't optional on exterior iron. And don't reach for a Zinsser here: BIN and Cover Stain are stain-blocking and adhesion primers, not rust-inhibitors, so they're the wrong tool for bare exterior metal. Spot-prime any rusty areas at minimum; prime the lot if it's been neglected.
For the topcoat, the decorator's route is an exterior eggshell over your anti-corrosive primer — Little Greene Intelligent Exterior Eggshell is excellent and comes in proper railing colours. The trade-standard alternative is an all-in-one metal system like Hammerite direct-to-rust, which trades a more utilitarian look and a narrower colour range for maximum convenience and durability.
Colour-wise, railings want to recede, not shout. Farrow & Ball Off-Black is the definitive choice — softer than a true black, reads as a deep charcoal in daylight. Down Pipe is the other classic, a grey-blue-grey that looks brilliant against red brick and stone. If you want something with a touch more depth, Railings (F&B) is literally named for the job — a near-black with a blue undertone. For period properties, Little Greene Lamp Black is a handsome deep blue-black.
Two thin coats beat one thick one — metal shows runs badly, and ornate railings love to pool paint in the corners. Use a small brush and work the paint into the detail. Pick a dry, mild day, avoid direct blazing sun, and don't paint if rain's forecast within a few hours.