Reinhard, @grimreaper
It's amazing at how much of a change it makes by adding the engine and exhaust staining. I have tried airbrushing on exhaust stains, and sometimes I have ruined the underlying paint in doing so. Because of this, I have switched over to using the Tamiya weathering decks, and sometimes finely ground up chalk in different colors. I use a moist cotton Q-tip bud to apply it. This way if I don't like how it looks, I can wipe everything off and start over again without the fear of damaging the paint underneath. Once I am happy with the looks, I seal it using a clear coat.
Judging from the way the exhaust streaks look on your Dad's Skyraider, he is much better at painting exhaust than I am... by a long shot !
Looking good ! Keep it going. 🙂