A PROBLEM OFTEN OBSERVED WITH PAINT FILMS is the development of domed blisters. Usually these bubbles in the paint film are the result of osmotic activity. Understanding the osmotic process will be very helpful in the investigation of the cause(s) or driving force with this mode of paint failure. Osmotic blisters may develop between the primer paint and base substrate, between coats of applied paints, or within the matrix of a single layer of the composite (single or multi-coat) paint film. Although in most cases the blisters are easily seen, it is possible to experience osmotic activity under paint films when a domed blister is not elevated enough to be visible. When this occurs on a steel substrate, typically, corrosion will develop under the film. The osmotic process may be represented by the following graphic. It is most important to appreciate that potential solvent molecules will migrate through the paint film one molecule at a time. One will remember that a collection of single, individual molecules is defined as a gas. For this reason, osmosis follows, and is controlled by, the general gas laws. A common definition of osmotic activity in paint films describes the migration of solvent molecules through the film and dissolving one or more solutes forming the blister fluid solution. This process then continues in an effort to dilute the blister fluid solution to the point that it has the same concentration (actually, same vapor pressure) as the outer contact solvent liquid. It is this differential in vapor pressures that drive the osmotic activity. Osmosis involves a semi-permeable membrane (as with a paint film), a solvent, and one or more solutes. To understand the potential interventions and investigation of the driving forces, knowledge of the basics of osmosis are extremely useful. Within this chapter we will review the chemistry and physics controlling the osmotic phenomena and present the analytical techniques that will allow one to determine the source, or driving force, of the osmotic activity. After determining the driving force one then has the ability to modify the paint formulation, application procedures, surface preparation, or service restrictions if any of these are shown to be the cause (source). If a coating failure has occurred one may determine where the liabilities lie as well as the extent of remediation that will be required to allow a true long-term fix of the problem. Problems associated with faulty application such as improper surface preparation are frequently claimed as the culprit. But, the inclusion of a water soluble thinner or chemical reaction under, or within, the coating matrix that produce water soluble materials are possible. Examples of some of these will be discussed later. Other potential sources of osmotic problems include deficiencies in the paint specification and unanticipated service conditions. Paint formulations that include pigments, resins, or additives that are soluble, or convert to a form that is soluble in service, can cause osmotic problems. On rare occasions manufacturing anomalies may lead to osmotic activity.