Yep. As said, nothing but oil based will bond to that. Even epoxy 2k primer has to have a level surface to adhere (which has to be sanded first for adhesion). The dimples in the hammered finish and features such as rivets make this tricky too....
I'd probably just overcoat with a smooth oil based paint....... However you might also find that the original hammerite over time will lift and peel anyway (some gates here did that over a 10 year period). That makes any sanding a lot easier