Fat over lean
As was mentioned in Methods of applying paint, when painting in the traditional method of layers, it is necessary to ensure the bottom layers dry before the top layers, otherwise the painting's surface will crack. The technically sound and fundamental process of oil painting to ensure the lower layers dry before the top layers is commonly referred to as "fat over lean." This process is necessary to ensure a smooth painting surface after the paint has fully dried.
Oil paint comes straight from the tube as fat, color pigments mixed with an oil medium. Adding medium to this mixture will make the paint fatter, which, in turn, increases its drying time. Lean paint is oil paint mixed with a solvent, traditionally turpentine. Lean paint dries faster than fat paint and thin layers naturally dry faster than thick layers.
Assuming uniform application of the same thickness, when lean is painted over fat, the lean layer will dry first. Unfortunately, as the fat layer beneath dries, the lean layer will crack. To complicate the situation, lower layers will absorb oil from the layers above them, causing the upper layers to become more lean. To counteract this natural tendency of the oils, it is imperative for each layer to be fatter than the layer beneath it. The drying times for each color will vary with each individual pigment as the amount of oil necessary to properly suspend each pigment will differ. A good practice is to use faster drying colors for the lower layers and keep the slower drying colors for the use in the upper layers.
When mixing a solvent or medium with an oil paint, it is important to use the smallest amount possible. A good ratio is to never exceed 1 part of additive to every 5 parts of paint. If your paint begins to wrinkle, you have most likely added too much medium. If you thin your paint to the consistency of water, the paint will become underbound and the oil will be separated from the pigment and no longer adhering the pigment to your canvas. Please note, fatness or leanness is not a measurement of viscosity but of the proportion of oil to solvent. Thicker viscosity does not necessarily mean fatter paint.
To summarize, if you complete your painting in a day or two, you may be able to use your paint straight from the tube and ignore the fat over lean process altogether. However, even when painting alla prima, it is a good idea to follow the basic fat over lean process. if you wish to take your time or only have the option to paint for brief periods at a time, fat over lean will certainly become your mantra.