What is the preparation for painting over a painted surface on a door?

To prepare a painted surface on a door for repainting, first clean the door thoroughly with soap and water to remove dirt and grease. Next, lightly sand the existing paint using fine-grit sandpaper to create a smooth surface and help the new paint adhere better. If the original paint is glossy, consider using a primer to ensure proper adhesion and coverage. Finally, wipe down the door to remove any dust from sanding before applying your new paint.