Garden gates

Can you help me with my discombobulation over options?
I have some metal gates across my drive which need tlc. They have been painted before with some kind of matt grey coating. The verticals are fine; the horizontals need a wire brush and a bit of emery cloth. Because I want a particular colour, I am using Johnstone's Weatherguard gloss paint for this. Can I paint directly onto clean, sound paint? If I sand off the blistered paint, will I need to spot treat with a) primer b) undercoat? If yes, to either of the latter, what should I look for ...zinc, phosphorous..bit of a minefield? I don't know exactly what the gates are made from - guessing they're wrought iron - decorative, but sturdy. Imagine there could be a bit of rust beneath the blistering.

Looking for a cost-effective but quality finish.

Many thanks for helping.

Johnstone's Weatherguard 6 Year Exterior Undercoat has been specially developed to be used in conjunction with Johnstone's Weatherguard 6 Year Exterior Gloss to provide an exceptionally durable and long lasting high gloss finish for exterior wood and metal.
http://www.johnstonespaint.com/exterior-products

Thanks, yes, I read this, but they would say that, wouldn't they? Is it like the upsell for x brand conditioner you see on the x brand shampoo bottles, or should one always use undercoat, even if there is clean sound paint beneath? Thankfully there's no rust or bare metal so I guess I can rule out primer. Thanks for bearing with me!