These railings, it appears, have over the years been painted black, white, red and blue, each new colour slapped on top of the flaking existing paint and corroded iron underneath.

I'd be tempted to come along in the night and shot-blast the whole lot back to clean metal and start again, first properly metal-priming the iron, or using Hammerite paint which is available in all of those colours and needs only a single coat. It might not look as picturesque though.