The principle to follow if you wish your oil paintings to last and not crack.
'Fat' oil paint is paint straight from the tube. Mixing oil paint with an oil makes it even 'fatter'. 'Lean' oil paint is paint mixed with more turpentine than oil.
'Lean' oil paint dries faster than 'fat' oil paint. A layer in an oil painting should therefore not be 'leaner' than the previous layer. If 'lean' is painted over 'fat', it will dry first, making that layer of paint vulnerable to contraction and cracking when the 'fat' layer dries. Hence the principle in oil painting of working 'fat-over-lean'.
Note that fat paint does not equate to or mean thick paint. If you thin paint with oil to a very dilute state, it would still count as fat paint compared to paint thinned with turpentine.
For more detailed information, read Oil Painting Techniques: Fat Over Lean
