Hi, Dave. First, be sure to remove the door and hardware then work on it on sawhorses out of the direct sun. Having the metal too hot may be a big part of the problem.
Sand with 150-240 grit paper on a sanding block then vacuum the surface and tack rag. You just want to get rid of the brush marks. If you don't, they will probably show through again. Brush or roller is your option but I would just use a brush for such a small area. If you are in a hot climate--say above 80 degrees--do what you can to cool the metal. I might even consider spraying some cool water on it then wiping it dry right before you paint. Check the paint can for the temperature recommendations. The other thing is to just let the paint flow on and don't try to brush it totally smooth. Today's paints will self-level quite a bit if you let them.