The key to painting metal properly is to ensure you first properly prepare the metal surface. Once the metal has been properly prepped, then primed, it is ready to be painted.
The surface of the metal must be made clean. For old, ferrous metal, use a wire brush to remove any rust from the surface. Follow up by wiping down the metal with a damp cloth to remove any dust and dirt.
New, galvanized metal must be washed with soap and water to remove any oily residue from the manufacturing process. Dry the metal with a cloth or allow it to air dry. Old galvanized metal, that shows signs of white oxidation, needs to have the oxidation removed with a wire brush or sander. Follow up by cleaning with a damp cloth.
The metal must now be primed. Use a primer designed for the type of metal you are painting, whether ferrous or galvanized. Apply two thin coats of primer, allowing four hours of drying time between applications. Once the second coat of primer is dry, apply your first coat of paint with a brush or paint roller. Allow to dry for four hours before applying the second coat. When applying the second coat of paint, ensure you are laying down the paint in the same direction to avoid brush or roller strokes and create a smooth finish.
If using a spray paint, shake the can vigorously prior to application. Hold the nozzle of the can 10 to 12 inches away from the metal surface. With an even, sweeping motion, spray several very think layers of paint onto the metal. Do not point the nozzle at one spot for too long, to avoid drips. Allow the metal to dry until the surface is no longer tacky to the touch.