Fat over Lean is a maxim used by oil painters which refers to the practise of using thick paint on top of thin paint.

When oil paint is layered, durability and endurance issues arise. To prevent cracks of the picture it is common practice to plan the painting so that the "fat", or thick oilier layer, is above the "lean", or thin layer.

Paints dry at different speeds and rates. In general the more oil present in the mixture the slower the oil binder will oxidize [in other words the paint drys slowly]. Paints having a higher concentrations of Varnish, Turpentine, or oil tend to dry at a faster rate.

Should you wish to ignore this rule, most likely the quicker drying paint on top of the slow drying paint will cause cracks, as the bottom layer will 'settle' and 'move' in the drying process. Something similar to foundation cracks in a building.

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SOURCE OF WISDOM
~capi crimm