Painting over 3D renders (Tutorial)

In this tutorial, I show how to paint over 3D renders.
Painting over 3D renders is something I do almost on a daily basis as a film concept artist.

3D helps a lot to establish perspective, lighting, textures and many other important details in your artwork, but there is always a moment when it feels like you need to put some paint work on top of it to push it further.

When is that moment exactly?
How to start painting over your renders?
What brushes to use?
How to improve storytelling by painting over?
It took me a few solid years to find well-balanced answers for those questions, which I`m sharing in this tutorial.