Swelling is defined as the undesirable separation of the coating from the substrate in the form of small or large bubbles. Blisters can occur if the coating is applied to a wet surface or is applied at high humidity. In places where bubbles appear, the coating peels off. Blisters also occur if the coating is applied at high air temperatures. Blisters that contain water are formed when water vapor penetrates through the pores on the coating, which condenses and “inflates” the coating. Surfaces that peel and where blisters appear should be sanded and re-painted.