Remove the door from the hinges if possible
Remove hardware, locks, and handles
Clean the door thoroughly with soap and water or a degreaser
Rinse and let the door dry completely
Sand the surface lightly with fine-grit sandpaper
Remove all dust with a tack cloth or damp cloth
Apply a rust-inhibiting metal primer
Let the primer dry according to the label instructions
Sand lightly again if needed after priming
Apply the first coat of metal-safe paint with a brush, roller, or sprayer
Let the first coat dry completely
Apply a second coat if needed
Let the final coat cure fully
Reinstall hardware and hang the door back in place