Again, I found it's all down to the brush.

using a normal brush was complete and utter rubbish even with black. I seemed to use loads of paint yet coverage was terrible and very streaky.
I even primered some and rubbed it down first to try and ensure that didnt happen after a friends cage didnt turn out well with hammerite.

Normal brush = shite.

Use those small flat square brushes. They're handy and quick too, and can be used off the handle for access into awkward spaces. Although for some corners a normal brush is still needed.