I always found that I need to put a primer down first and then use a wide enough brush loaded with paint which should lay a nice covering coat on the first swipe. To little paint on the brush will cause streaks.
I use a medium/dark gray primer in a spray can, then dry brush Testors aluminum (a flat aluminum) to give a weathered steel look. There is a tremendous variety of appearances to steel, depending on alloy, and time of weathering. Polished steel, especially stainless alloys, are next to impossible to brush properly, but weathered steel of many alloys can be done by various techniques, the above only being one.