Geoff why bother painting it ? Galv on it's own will look good for years & years ..
There's not many paints that stay on galv no matter what you prime it with unless it is an oilrig North Sea resisting type paint . .
Paint does not stay on galv very long then it looks a dappled mess , so you then have to strip it off or spray over it as out local council has found out .. 400 mtrs or more of 2 mtr high park fencing , took a couple of months to paint black with gold painted finials , 18 months late it needed doing again .. 2 years later it needed doing again , now it needs doing yet again but they have left it this time round .
I personally would rather a painted finish outside a domestic dwelling.. definitely a galv finish everywhere else.. The company that I got the galv done with have supplied paint that is meant to be suitable for painting over galv with so only time will tell