The main reason you get (colour) banding in Software like Photoshop is actually because your layer masks have zero texture, when using a gradient or soft brush. Thus they introduce sharp lines on certain surfaces - typically the sky in your image!
This can be totally avoided by adding a bit of noise to the layer mask!
I wish I had known this earlier, it's such an easy fix to avoid a massive problem and it makes you wonder why PS is not doing this automatically when detecting banding!