According to Golden Paints, a leading manufacturer of acrylic paints and mediums: "The Two Drying Stages of Acrylics
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."
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