Ditto on JSM's comment. The only place I would go to the metal is where you have a rust spot showing up. There I would sand down to the metal to determine if it was a surface rust caused by a flaw in the original paint or a rust thru starting. In either case, you need to do something about it. In the case of a rust thru starting, you need to cut out the bad spot and replace the metal. In the case of a surface rust caused by a defect in the original painting, you need to apply a rust converter/killer to that area before continuing.
The existing surface needs to be free of wax and contaminants plus lightly roughed, primed and wet sanded to make it as smooth as possible, (get a painting book if you plan on doing it yourself, you can find them in the Auto books section of any book store). The finish will only look as good as the prep work looks.
Good luck,

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