Repaint metal railing

The usual routine is to scrape, brush, and sand until all loose material is off, then prime and paint (or use a self-priming paint). In this case it's not clear whether there's a bonding problem with the black coating or whether it's just very old and neglected.

It's possible that if you do this job without a complete strip to bare metal (or the white undercoat), the black layer will continue to delaminate. That's the risk you'd take and how you proceed is up to you. If you want to strip it you have a few options--chemical, sanding, media blasting, etc. It comes down to your will and your budget.

Good luck.

Since the surface is already painted I would not use a primer but getting that loose paint off will be important or the same will happen to this coat of paint.

You could use a paint scraper or corse sandpaper to help clean the loose stuff up but I would use an angle grinder with a wire brush , I use cup brushes with twisted wire to clean even badly rusted metal.

Once the loose paint is gone and the surface wiped clean any metal paint would work I like spraying but it can be brushed on Don’t over work the paint or the brush marks will show.