"Acrylics dry as the vehicle that carries them, mostly composed of water, leaves the film. As water evaporates or is absorbed by the substrate, tiny acrylic polymer spheres are forced into ever closer contact. Eventually they are crowded so tightly that the spaces between them create capillary forces, and water is pulled from the paint film. This capillary action packs the acrylic spheres against one another in a honeycomb-like pattern, and they begin to form a continuous, cohesive film. As this occurs, the polymer spheres, composed of long chains of acrylic, actually deform and partially combine with one another in a process of film formation called coalescence
The drying of acrylic paints occurs in two very different stages, hence drying times must be thought of in two different time frames. The first stage, a relatively short period of time, results in the formation of a skin over the surface of the paint. This is the time that it takes for acrylics to "dry to the touch". At this point, the flow of water towards the surface is no longer sufficient to keep the paint film wet. Very thin films can feel dry within seconds, while thick films may take a full day or more to skin over.
The second stage of drying is the time for the entire thickness of the film to be thoroughly dry. That is, the time required for all of the water and solvent (used as freeze-thaw stabilizer and coalescent) to evaporate and leave the film. This is a most crucial time frame, as the ultimate physical properties, such as adhesion, hardness and clarity, do not fully develop until the film is near complete dryness. For very thin films, this time may be a few days, while films of 1/4 inch thickness or more will take months and even years to be completely dry.
Many artists are not aware of this more lengthy drying time. This is the reason that one may find that a rather thick layer of paint has not adhered to the surface when tested a day or two after application. This same layer of paint will also seem very soft. The skin may have dried sufficiently, but the paint in the center is still wet. Regarding development of clarity in gels and/or mediums, one can allow a painting to clear, store it away and later notice that it has become cloudy. The film may have only been partly cured, and is soft enough to allow moisture from the air to penetrate, turning it slightly milky again. Given enough time for more complete drying, these properties should improve dramatically."
Well, I was going to post about the Clegg v Huhne Lib Dem leadership contest. But I thought this article on paint drying was far more interesting.