How to clean inside wrought iron handrail on saircase
Assuming the railing is painted, you should use a brush that isn't too abrasive. I'd start with soap and hot water along with a scrub brush like you'd use on pots and pans (with natural or synthetic fibers). You can add chemical household cleansers if needed.

To fully refinish the railing, you'll want a wire brush. Use this to clear off any flaking paint or flaking rusty areas, as well as scour the entire surface to prepare it for a new layer of paint. Clean with a mild solution of water and soap, let dry, and cover with a primer/paint designed for metal. I'd recommend a spray, because you can find primer+paint products designed to seal over light rust, which is often a concern on iron railings. But you can also use a brushed primer/paint as long as the directions indicate it's OK for metal.