I would pressure wash it real good and when it is dry, wash it with metal etch keeping it wet for 10-15 minutes. Then wash it again with your pressure washer before the metal etch drys. Wipe off with clean towels and blow with compressed air until it is dry. I like to run a fan on it for a while to make sure I have it real dry. The bare metal will have a green tint to it when dry. Ready for paint.
