With the Leyland product or the Johnston's Smooth metal paint https://www.johnstonestrade.com/product/smooth-metal-paintI've bought Leyland trade black. I'm under the impression it can go straight on with no primer as long as the steel is in decent condition?
If is new and unpainted before, I would prime it with red oxide primer or any metal primer. If it is new galvanised steel you will also need to T-wash the steel first before the primer.