In reply to Enty:
If it were new, you'd apply an etching solution, followed by a rinse and wipe with acetone, followed by a primer, followed by your paint of choice.
As it's tarnished and tatty you have to add a few days of elbow grease, rubbing it down to remove all tattyness and tarnish, or a session sand blasting it.
Etching and a good (ie expensive) paint mght give you decent results following a good clean, but hardly worth the cost. Look up 'POR15' paint (they do the whole etching solution, optional primer and top coat system) or an industrial epoxy paint.
There's a reason why you don't usually see painted galvanised steel and that is why