I have exterior french doors on my patio and they are in strong sun light for over half the day. I had them put on about 6 years ago and I never treated them. Now I have a white power that comes off on your hand. What can I use to treat and paint them and how to go about doing it.
The first thing to do is a good washing. This will remove most of the oxidized primer or paint. This is the powdery residue you noticed. Now the surfaces need a light sanding and another wash or wiping down with a liquid sander.
After the prep your doors can be primed, with a good acrylic primer, and painted with a quality acrylic paint.