Oh dear Sparxxxxy,
Talking to another painter today, told him your situation and my solution. He called me a bloody old fart and told me time has moved on, bloody little upstart.
Evidently, Zinnser 123 is the way to go, (I'm sounding like Paintycait now.)
Must put my hands up, I haven't painted galvanised stuff for a loooong while.
Mordant solution is an etching pre-treatment for surfaces like galvanised metal that has not been weathered. After degreasing the surface, you brush on the mordant solution, which will change the colour of the surface, (which also points out any misses), and etches it to give the primer a 'key', wash off and allow to dry; then apply chosen primer.
But bugger all that, get some Zinsser 123 and whack that on.