Flooding is detected in the form of changing the colour of the wet paint film after application. It results in a uniformly coloured paint film with one or more pigments enriched at the paint surface. The separation of pigments in wet paint film occur because of different pigment densities and particle sizes, accordingly different settling tendencies of them.
In the flooding defect, as a result of the pigment migration towards the film surface, the concentration of pigment on the surface becomes homogeneous and the colour appears darker or lighter than expected. Due to the fact that the colour change is uniform, flooding is not always easy to detect, especially in test panels applied in the laboratory. However, the remarkable colour changes can then take place in the application lines; therefore, flooding tests are useful. The flooding can be detected by applying rubbing test to the wet paint film. With the rub-out test, it is determined whether the colour in the tested area is different from the untested area by rubbing with a finger on the wet paint film.
Fig. 1 Rub-out test
Flooding and floating defects are often confused with each other. The difference of floating from flooding is that colour differences occur in different parts of the panel due to non-homogeneous distribution of pigments as a result of pigment migration towards the film surface (Figure 2).
Fig. 2. Illustration of floating and flooding of pigments in a wet film
Thick films, low viscosity, solvents with low evaporation rate, slow drying (or hardening) resins, low "pigment/binding" ratios and any factors that cause the film to remain wet for longer are the conditions that increase the flooding tendency.
As a precaution, the use of thickeners, faster solvents and more reactive resins help to increase viscosity. The similar density of the pigments used together, the primary particle sizes and the level of dispersion also have an active role in preventing flooding.