I would recommend you look into multipass rendering to get some passes out that you can composite to get the falloff of clear coat paint (you could even use your clay rendering and composite that into your material rendering) but you will need a rendering with more highlighted areas.
Oo.. i remember you posted that plane you were modeling a while back it looks even better now!
Maybe you could try to place it on a airfield, just a simple modeled and textured background and then composite in clouds in the Maxwell sky on your rendering..
That last plane rocks!!! Maybe the metal material on the engine could use some work. Seems like it has too much roughness. Maybe different parts of the motor could have different shinyness?
He, nice too see my "HDRI" from the a place in the near of my hometown again. Crazy, now at the place will be placed virtual CG objects.
In the image with the visible background I would try to place the MXI sphere a little bit lower in the scene. Than, not so much of the gold-brown leaves on the ground are viewable and catch the eyes.
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