One of the components of a polyamide epoxy contains a low molecular weight epoxy resin, while the other component contains a low molecular weight amide. When mixed together, the two components react to form a tough, densely cross-linked coating with good moisture and chemical resistance, and excellent adhesion to blast cleaned steel. Despite their good properties, polyamide epoxies have been observed to blister occasionally when subjected to prolonged water immersion. Often, such blistering is osmotic in nature, and is caused by the presence of water soluble material either on the steel surface or trapped in the coating. These materials could be salts that were inadvertently painted over, or even water soluble solvents employed in or added to the coating.
All coatings are permeable to water, some more so than others.
Once a small amount of water permeates the epoxy coating and comes in contact with the water soluble material, it will dissolve it. Although there may be very little water soluble material within the coating, there is also very little water. As a result, the concentration of the materials within the micro-droplets of water inside the coating is high. The result is a liquid filled blister. An interesting consequence of osmotic blistering is the fact that pure water is usually much more aggressive than brackish or contaminated water, at least if the water soluble contaminant is a salt.