As a starter, let us learn the basics of painting CDs. Gesso is an all around medium that you can use to prime any surface. It prepares the surface so that you can apply paint or any medium into it. For comparison, I used two types of gesso, a liquid, thin gesso and a thick, paste-like consistency gesso.
Photos below will show the difference between using liquid, thin gesso and thick and paste-like consistency gesso.
Thick Gesso
Liquid gesso
I am not sure if the first coating is thin because of the watery brush that I used. But notice that after several coating, you will achieve the same effect on the CD.
Based on experience, liquid gesso is better to use on papers and canvass and thick gesso can be used on non-porous surfaces like plastic or even metal.
There are even other types of gesso out there but you are free to experiment which one suits your preference.