There are three methods you can use for painting metal doors.
The first is a simple brush. While this is the least expensive method and likely the most popular, it is not the most effective. Each brush mark shows up quickly on the door’s smooth surface. Using a professional quality paintbrush will help.
The second method is using a roller. This is easy, requires little expenditure, and little training. With a short nap roller cover, and the right size of roller (use a 4” roller if there are many decorative indents or rails on your door) it is possible to get a very smooth finish by rolling.
The last method produces the best results on metal doors. Spraying gives a finish that is truly smooth. However, using a paint sprayer is expensive, and requires a bit of know-how to use effectively.
Read more: Metal Door Painting: Start to Finish | DoItYourself.com