I have often read about the need to remove mill scale before blacking, either by angle grinding, grit blasting or leaving it to rust for a few months. But outside narrow boat building, what does the steel fabrication industry do? Angle grinding and leaving it to rust don't look to be credible solutions, and I can't believe that fabricators pay the extra cost of blasting in most cases. Which suggests they largely get away with just a degrease, primer and paint.
I have been told that the top notch boatbuilders used pre-blasted and primed plate for hulls, but prefer plain steel, primed after fabrication, for cabin structures as it is easier to get a good line and finish that way.