This old maxim concisely expresses how, generally speaking, a successful and durable painting is developed. I say “generally” because there are instances of great painters bending this maxim to conspicuous effect, but nevertheless to ignore it is to court disaster. It could be clarified by paraphrasing it as “Flexible over Inflexible”.
Paint as it is squeezed neat from the tube can be modified by adding to its oily binder. If a diluent like turpentine is added then the binder is thinned in what is called a “vehicle”. When wet such a paint is more fluid in handling, but when dry, most of the vehicle has evaporated, leaving the oily binder far more sparsely distributed through the pigment particles. As a result, the glossiness of surface caused by the dry oil is proportionately reduced, as is the physical strength and flexibility supplied by the gluing presence of the oil molecules, called polymers. A dry layer of such a diluted paint is, in consequence, matted in appearance and less flexible and more brittle in structure. It is said to be “lean”.
If, however, we add to the tube colour more oil, or a medium which contains oil and possibly resin ingredients, then we have added to the proportion of binder relative to that of the pigment. Assuming that whatever medium we use does not contain much of an evaporating vehicle, the resulting paint mix will dry with a greater amount than previously of the oil which gives it a surface gloss, and gives the dry film some flexibility. Although the pigment particles are thereby being more sparsely distributed than before, the surface quality imparted by the increased gloss usually optically intensifies their colorific strength. Such a glossier and more flexible paint is said to be “fat”.
As all painters realize, the rapid evaporation of the vehicle makes lean paint, with comparatively little oil, dry more rapidly than the same unmodified paint from the tube, and far more rapidly than fat paint, which has more oil still. The process of drying of oils is chemically complex (of which more later), but it entails quite a lot of initial rapid expansion of volume, followed by a slower, more gradual contraction (sometimes plotted as the “drying curve”). Paint moves as it dries, and the more oil it contains, the more and the longer it moves.
By now the sense of the maxim above must be clear: if you paint first with fat, flexible paint, and then paint over this with lean, inflexible paint, you are laying up the strong possibility of structural problems later. Of course, the visual result may indicate much more quickly that in some way you are abusing the material; the lean paint will probably dry as a mat, chalky and even resisting (i.e. “beading up”) film on top of more colourful and glossier underlayers. Later on, the lean paint, having quickly reached the end of its drying cycle, might be moved around so much by the still drying and moving bottom layers, that it will crack and shell off in pieces. Not a happy prospect.
So as a general rule, you should work over your layers of paint in the broad direction of: turpentine-thinned paint-neat tube paint-medium or oil enriched paint. In this way the hierarchy of drying speeds is respected, and the appearance of the work should not disappoint.
As a final observation, there are a few paints with very high oil content (i.e. more than average quantity of oil needed to make the pigment or dye component into a stable paint.) which, even as neat tube colour, dry very fat. If using these in underlayers, it would be wise to thin them first on the palette.