Yep, I see that. Fenders answer wasn't there when I typed mine.
If Hammerite has a primer function included then there is no need for additional primer, however you're still applying a primer function to the metal, which is essential.
I assume Chip wants this item in the garden? If so then it really needs primer else it will rust through. I have black painted galvanised metal in my garden, it didn't have a primer on it and the whole bottom shelf has rusted into powder. The other shelves all have holes in them.
When COVID is over i'll be taking it to the tip.