iv always used Hammerite special metals primer on bare metal and its been a success tbh, iv used it on various galvanised roller shutter doors, garage doors and gates over the years and its performed great.
i used it on my own gates approx 4 years ago followed by 2 coats of Coovar ebony black and they still look great with no cracking, flaking or peeling etc.
iv seen no end of galv garage doors flaking due to incorrect priming