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Michael, the concept of "fat over lean" is actually something quite apart from how dry an underpainting layer is before continuing to overpaint, although that is certainly a factor in painting in a layered technique.
Essentially, "fat" paint contains a lot of vehicle . . . that can mean its "chubbiness" results from the addition of mediums of all descriptions, but "fatness" is also a function of the pigments involved. Certain pigments, (e.g. ivory black) are very absorptive, requiring a lot of oil to reach desirable working consistency as paint. Other pigments, notably most earth colors, have a low absorptive index, and so they are naturally "leaner".
The relative dryness of underlayers as one progresses with a painting is a factor in good painting practice. Continuing to work on layers that are touch-dry but not dry all the way through promotes build-up of paint layers that become a homogenous unit by forming a chemical bond through solvent transfer.
It's a good idea in general to avoid using cobalt or manganese siccatives in particular, because these compounds promote rapid drying from the surface inward, causing paint to "skin over" and effectively sealing off paint that is not yet thoroughly dry. The difference in drying rates and shrinkage can cause crazing and other defects.
The reason "good" turpentine is a better solvent choice than MS or OMS (both are petroleum distillates similar to kerosene, but more highly refined) is that hydroxides in turpentine react with acids present in linseed oil. The reaction binds ambient oxygen, which promotes thorough drying of paint films from the "inside out" as well as from the top down.
Turpentine is in trouble. In many circles, it is seen as a horrible, smelly, toxic substance akin to nuclear waste, while OMS is tolerated as being "safe" on the basis of being odorless . . . which is rather a dangerously uninformed point of view, since both turps and mineral spirits have a near identical vapor pressure. This means an open container of either will emit the same amount of hydrocarbons into the studio atmosphere through evaporation. Safe practice demands that all solvent containers be covered, and that studios have adequate ventilation.
For the past couple of years it has been nearly impossible to obtain "good" turpentine, i.e., the stuff that is distilled from tapping the gum of living conifers, a process similar to collecting maple syrup. Pure Spirits of Gum Turpentine is water-clear, and smells clean, like a pine forest after a rain.
What has supplanted "good" turpentine in hardware and paint stores is a foul-smelling distillate produced mostly in the orient by grinding stumps, limbs, slash and forest waste and steam-cooking the mash to obtain a liquid with the chemical properties of pure spirits of gum turpentine, but with other adulterants and a fair amount of free water. It is the camp-follower of deforestation, reeks of creosote, and should NOT be used at the easel for any purpose.
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