Galvanised hand rails - to paint or not

1. It should be painted. Anyone using the rails will rub away a tiny bit of zinc each time,and eventually it will get through to the metal underneath.
2. No idea about priming galvanised iron.
3. *NEVER* paint a handrail black! No one will be able to use it in summer after the sun has been on it as they won't be able to hold on. I had a handrail made for the three steps up to our front door. It came already painted black, in early August. Within a couple of days I used white Hammerite to overpaint it as it was unusable when black.
4. Work out the surface area of one post and the single rail length to the next post, and multiply accordingly for one or more coats.

Goggle Ai gives this information Painting galvanised steel rails in the UK requires meticulous surface preparation—specifically cleaning with solvent/degreaser, using a T-Wash (mordant solution) to etch the zinc, or allowing it to weather. Apply a dedicated galvanized metal primer (e.g., Zinsser Bulls Eye 123 Plus) followed by a high-quality acrylic or epoxy topcoat.

Zinsser gives these hints (obviously recommending Zinsser products)

Black is possible the wrong colour for outdoors, as other have indicated the railing may get really hot in summer and this heat may cause paint to fail sooner. Depending on age also consider failing eye sight and perhaps needing a better contrast to see safety equipment in poor lighting conditions.

I don't know how the following would work with your railings but I had great success using a sock on my ornate stair banister rails. Instead of using a paint brush use a heavy duty woollen sock on your hand. First put on a thin Nitrile disposable glove, then put the thick sock over the glove. Socks sold as work socks are usually quite cheap. Pour the paint in a paint tray and dip the palm of your hand in the paint. You hand will fit around a pipe better than using a brush and is much quicker. However the Zinsser paint is claimed to have a fast drying time so may not remain liquid long enough for this method. The sock may use more paint than just using a brush.

Galvanised iron gates, scaffold poles etc. seem to be OK unpainted.

Leave it. Galvanised steel will last for decades and does not need maintenance - if you paint it you are starting a maintenance cycle that is unnecessary. You are very unlikely to remove the zinc layer with normal usage. Painting it is a pain and requires ageing and/or special primers ... even then there can be problems.

Indeed. Until the zinc coating is damaged, they last indefinitely. I think the decision is mostly aesthetic.

Not within a timeframe any of us would care about, I suspect. And not extensively.

Or get the rails powder coated (specialised effort)