Linda Ciallelo: I just realized that no one answered your question about the "fat over lean" rule. This is one of those unbreakable rules that should be followed unless you don't mind your painting surface cracking over time.
Sometimes it helps to simply remember that all mediums are considered FAT. And here is a more detailed explanation that I hope clarifies...
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. Again, 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 (dull) are LEAN.
Some other painters (Rubens sometimes worked this way) often apply thick opaque paint as the initial layer and add transparent glazes to modify that underpainting. The "fat over lean" principle applies to this too. I did an underpainting example at:
http://forum.portraitartist.com/show...=&threadid=794 that illustrates this.
"Fat over lean" applies to all methods of painting in traditional oils whether direct, indirect or when using a combination of methods.
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