I just heard back from Winsor and Newton regarding this inquiry:
"If you add thinner to Artisan Quick Dry Medium is this mixture leaner then Artisan Quick Dry on its own.
Is artisan Quick Dry Medium on its own leaner then the paint."
"Thank you for your inquiry regarding the fat over lean rule.
Basically, fat over lean is just another way of saying "more flexible layers over less flexible layers". A paint layer that has a higher quantity of oil in it remains more flexible and is therefore known as a "fat" layer. A paint layer that has less oil in it remains less flexible and is therefore known as "lean". If
you place a less flexible layer of paint on top of a more flexible layer of paint it can be a little like painting on a rubber band. If you apply paint to a rubber band, allow it to dry, and then pull the rubber band, the paint layer will crack and flake off the rubber band. The same thing will happen to less flexible layers of paint over more flexible layers of paint - as the more flexible layers of paint move while drying, the less flexible layers will try to move with them, but will ultimately be unable to, resulting in cracking. The safest way to keep to the fat over lean rule is to increase the amount of oil (in the form of a drying oil such as linseed, or a medium) added to each subsequent paint layer.
Adding thinner to the Artisan Quick Dry Medium will make it "leaner" than Quick Dry medium on its own, because the additional
thinner will dilute the oil in the medium. Answering your question regarding Quick Dry Medium being leaner than the
actual paint is a bit trickier. Different colors may contain different amounts of oil, due to the nature of the pigments that are contained in the color. Some pigments require a greater amount of oil than others, for even dispersion. Frankly, I really can't give you a definite answer on this one, because I don't know the exact recipe for each color (this is proprietary information and they won't even share it with me!) and each color would have to be looked
Janet, I hope this has helped somewhat. Please do not hesitate to contact us again if any of the above is confusing or if you have any further questions.
Winsor Newton Technical Support"
Winsor & Newton Thinner
Lukas Berlin Quick Drying Medium
Winsor & Newton Fast Drying Medium
M. Graham Walnut Alkyd Medium
Linseed Oil (All brands I would think)