Either paint it or replace it. Painting is much cheaper of course. All loose 'powder' will need to be removed and all mold, mildew and dirt removed, then prime and paint. A decent paint store expert (like Sherwin williams, benjamin moore, etc) will be able to advise you as to which of their products will be best for primers and paints.
I seem to recall some type of paint that was designed to go over the 'powder' without removing it. I would stay away from gimickey stuff.
A pressure washer may be just the thing to prep it for primer and paint.