The UV or sun is very hard on painted metals. Over time the top layer of paint gets "oxidized" and fades. You will feel and see on your hands a white chalky powder when you run your hands on the door. This is the failed paint on your door. You can remove it with a chemical and a soft brush. You have to be careful not to remove all the paint on the door by leaving the chemical on too long or brushing too aggressively. You can see the difference in the picture here, the dark color is cleaned. Wow!