Rough up the entire surface of the safe with the wire brush. Some areas may require more work than others because of paint bubbling or rust. Get the hard-to-reach areas with heavy-grit sandpaper.
Tape off all areas the paint can over spray onto. This is important since spray paint will travel through the air. Safes also tend to be heavy and you may not be able to move your particular safe.
Spray the paint onto the safe. Hold the can 12 inches from the surface and move in lateral movements using small, short bursts to apply the paint. This will limit the possibility of drips and runs. Allow it to dry, then add a second coat.