All garage doors, whether they are metal or not, need to have their metal parts (springs, hinges, etc.) oiled regularly with WD-40 or another lubricant to keep them running smoothly and to prevent rust. However, when you are dealing with a steel garage door, you may encounter more serious rust problems that present cosmetic difficulties as well as ultimately interfering with the functionality of your door. If you have rust spots on your garage door, then you must remove them immediately and take steps to prevent them from returning. If the door is extremely rusty, you may have to strip the entire area. However, in many cases you can spot-treat the problem to prevent it from growing.
Wash the entire door. Use the spray bottle to apply a mixture of hot water and 5 drops of liquid detergent. Wipe down the door using a cleaning rag, and attack tough spots of grime with the scrub brush. Removing dirt and buildup will help you identify rusted areas so that you do not have to do any more sanding than you have to. Have someone go behind you with a clean, dry rag and dry the door as you wash so that you do not risk any flash rust creating a bigger rust problem.
Sand off the rust. This will likely involve some pretty serious "elbow grease" since you will also be sanding off the paint in the process. Sand off the entire area that contains rust, and make sure that your sanding extends about 2 inches past the visible rust line to ensure that you get it all. You can do this by hand or with a power sander if your door is flat enough to make this effective.
Clean off the sanded areas. Use the bristle broom to sweep off any dust from sanding, then wipe down the entire area with your detergent solution again and dry the area completely. This will help your paint stick to the metal more effectively.
Apply the primer. You can do this using a rag or a paintbrush. Apply a thin, even coat that slightly overlaps the painted areas that you did not sand. The oil-based primer will inhibit water and moisture from coming into contact with the metal garage door again. You can apply two coats, but wait at least 90 minutes in between coats to let the primer dry.
Paint the garage door. Use your matching paint to cover up the primer and mask the areas where you sanded off the rust. You can apply as many thin, even coats as necessary to make the sanded parts blend in with the rest of the door. Just allow about 90 minutes drying time in between each coat.