If exposed to direct sunlight, the UV rays will completely fade the color, but it will not lose the purpose that it serves...the rust will NOT come back.
You can put a base coat of paint down within 4-12 hours of spraying the POR, if longer than 12 hours, scuff it, and use a self Etching Primer to cover the POR, then use your regular desired primer, then your base/clear or single stage paint...whatever paint you perfer!!