Degrease, sand well with 80 or 120 paper in a da sander (if you have one) or by hand if you haven't.
Clean and degrease again and apply an epoxy primer before a top coat of your choice.
My go to primer for galv is Jotun Penguard HB but it only comes in 5l so probably too much if it's a one off job. Lots of other epoxies will do the job and they are better than etch primers.
T wash and modant solutions work, but are not as good as an epoxy primer, however if you are painting the rest with hammerite it will probably be more than good enough for your job.