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    There's a good reason that paint manufacturers warn against thinning waterborne traffic paint, commonly used on airports. The FAA echoes the warning in AC 150/5370-10F, P620-3.5, "The addition of thinners will not be permitted". So why do some equipment manufacturers tell their users to thin the paint if they're having trouble spraying it? Well, the obvious reason is that it sprays easier. But the result is a paint film lacking the properties that make a waterborne coating durable.

    There's a lot more science involved in paint than most of us would think, and the following is a simplified explanation of what happens when the paint meets the pavement.

    Waterborne traffic paints are made up of pigments, acrylic polymers, surfactants, fillers, resins, coalescing agents, water, and solvents. Percentages of ingredients, viscosity, and other characteristics are prescribed in the paint specification for TT-P-1952E.

    When the paint is applied, a volume reduction occurs as the solvents (mainly water, but also ammonia and methanol) evaporate, and through absorption by the substrate. The decrease in volume causes the particles to move closer together. The most durable coatings are those where the particles made up of pigments linked to the polymers are evenly disbursed. As the particles start to deform, coalescing agents in the paint keeps the film "open" for an orderly evaporation process; and finally there is a homogeneous, coherent film that is complete during the curing stage, about 24 hours under dry weather conditions.

    So it is evident that any change in the chemical composition can alter the balance and interaction of the pigments to the polymers and prevent the coalescing agent and other components from performing as designed, sometimes resulting a relatively brittle, less coherent film that may not resist ultraviolet light (UV) and which may cause premature degradation. Additionally, glass beads that have coatings to enhance adhesion to the paint film may not anchor well in the altered coating as a result of the imbalance in the paint formula.

    We have heard complaints from folks in every discipline that "waterborne paint doesn't last!" In our experience, the waterborne technology specified for airports is excellent and can be extremely durable, assuming manufacturer guidelines are followed.

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