Let's create a new layer. In the Colors window, create a blue colour with R = 212, G = 232, and B = 255 (1). Invert the Primary and Secondary colours, and select the white in the palette (2). We have now white as primary colour and sky blue as secondary colour.
Choose Gradient in the Tools window. By clicking at the starting point (bottom) and then moving up while holding the button down, the gradient appears, contained in the selected area, white down, blue upwards.
Now, to avoid a too sharp cut out, after deselecting the sky, let’s do Effect / Blur / Gaussian Blur / 2 (exact value to test, say 2 to 4 depending on the image size, but 2 most often gives excellent results), which has the effect of blurring the “sky” limit by blending the blue pixels with transparent pixels.
Applying the gradient, then the Gaussian blur
For safety reasons and also to reserve the possibility to go back on modifications, it is better to save the complex image to the proprietary format PDN. This can be used, for example, if the mask is subsequently discovered incomplete and the “sky” pierces the object being photographed!
Now we just have to delete the Mask layer, and to merge the Sky layer into the Background layer.
We can also simply do Image / Flatten, but then we get no control on the order in which the mergers or deletions are going to take place (it may have its importance).
Effect of the blur applied to the sky: if you zoom in on the limit between the model and the sky, you will see that this limit has a gradual, not too sharp transition, which would betray the “montage” and would be not really aesthetic.
General view enlarged 2 times.
“Magnifiers” × 4; at the top, without blur: the aliasing is visible; at the bottom, with blur.
Let's finish by saving to JPEG. Here is the result.
Note: for car photography, as here, it is a little more complicated, because you must reserve a different treatment to the windows. They are certainly transparent, but absorb some light; it is therefore necessary to darken a little the light crossing them. However, specular reflections, ie reflected by a very smooth surface like water or glass, must be kept. That's what I did here (see the windscreen).
This is the end of this little trip in photography. I don’t pretend to hold THE truth, but only some experience. I do not claim to have explained everything in a clear and exhaustive way! I left you a little work… Enjoy!