"Fat over lean" explained
 

In indirect painting ((Leonardo style - painting from dark to light) the artist builds up three general layers of paint and medium.

Bottom layer or layers: A color is used to block in the painting. The paint in this layer is often thinned with turpentine and a small amount of medium. This layer is lean.

Middle layer or layers: This is where opaque colors are introduced. The medium can be thinned with 10% to 20% turpentine.

Top layer or layers: A glaze layer that modifies the opaque colors and makes the surface very rich. No thinners are added to your medium in this layer. This layer is fat.

Paintings built in this manner follow the "fat over lean" rule. ALL PAINTING MEDIUMS ARE CONSIDERED FAT so thin your medium less as you work from the bottom layers to the top ones.

Oil paints that look shiny are FAT to begin with. And oil colors that look more matte are LEAN.

Some other painters (Rubens style) often apply thick opaque paint as the initial layer and add transparent glazes to modify that underpainting.
No matter what painting method or combination of painting methods that are used, the "fat over lean" principle needs to be respected.