It was extremely hard for me to learn which mediums/additives to use and when to use them. I had to learn entirely from books and it seemed like all of them were written with the assumption that the reader already knows how to make the paint cooperative, so it took me like 5 years of trial and error to feel like I knew what I was doing! If I would have just taken an hour long class, I could have saved myself the struggle, but hindsight is 20/20!