Fill a bucket with hot water and add dish soap. Stir the mixture to make the water soapy. Dip an old rag in the soapy water, and use it to wash the door. Oily fingerprints, dirt and other residue can prevent paint from adhering to the door properly. Dry the door thoroughly with a towel.
Fill all dings, scratches and dents with the proper filler for the door's material. Use auto body filler for metal doors and wood putty for wooden doors. Apply the filler to each blemish with a putty knife, and smooth over the filler so it is slightly above the door's surface. Allow the filler to dry the amount of time specified on its packaging.
Sand the filler with fine-grit sandpaper until the filler is even with the door's surface. Sand the entire door to allow a new coat of paint to adhere to it better than if it is not sanded. Use a damp towel to remove the dust created by sanding.
Spread newspapers underneath the door, and paint the entire door. Painting only the parts of the door you fixed may result in color variations, but putting a new coat of paint on the entire door will refresh the door's appearance. if you prefer, remove the door from its hinges and place it on a couple of sawhorses to give it a coat of paint.