ReferenceMichel used this reference shot of a Russian Church's cupola as a starting point. He was especially looking at how the metal roof reflected light.
Base ColorIn photoshop he created the cupola with the proportions he wanted. Just like traditional painting he started with a brownish base color so that no white might peak through the rendering. Using the eyedropper tool set to 5x5 pixels he sampled the original photo for colors and airbrushed the highlight first.
True GritPlain photoshop airbrush has a tendency to look mushy. So to add some feeling of grit Michel painted some loose brushstrokes in oil and scanned them in. He then changed the blending mode to Hard light and lowered the opacity a little.
FinishingHe then used the dodge and burn tools for highlights and accents to the now distressed copper surface. The porthole windows were later added to finish off the the cupola.