Sagging, running, curtaining, and slumping.

The terms sagging, running, curtaining and slumping describe unsightly gravity driven flows on vertical surfaces. As is common with paint terms, there is not a consensus as to exactly what is meant by each term, but their appearance tends to define them. They can be very obvious such as tears or fingers of paint running down a wall or the side of a cabinet (sags or runs). They also can be subtle such as a slightly irregular line or curtain of basecoat or clear along style lines or window sills of a car or truck. Sometimes the sag would not be noticeable except that fine popping occurs in the thick area. Slumping tends to be thought of more in connection with plastering and cement work where a whole area drops down, but it can occur with paints, especially thickly applied ones such as bridge and maintenance paints. I will lump all of these defects together and call them sags. These defects are caused by too much flow (too low a viscosity). This excess flow can occur on application (cold sag) or, with baked coatings, in the oven (hot sag).

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Sagging can be reduced or prevented by raising the low shear rate viscosity of the paint and/or applying thinner coats of paint. The film viscosity can be raised by using faster solvents or by introducing thickeners or thixotropes. The latter are additives such as fumed silica, treated clays, microgels and castor oil derivatives that form physical networks. Associative thickeners in waterborne paints serve a similar purpose. Structures form after application to reduce flow and also modify viscosity-temperature behavior to prevent sag in the oven. Unfortunately, they also can prevent flow-out and leveling on horizontal surfaces, so amounts of the additives must be chosen carefully. Hot sag also can be reduced with the addition of materials such as waxes and certain castor oil derivatives that melt in to raise dissolved solids and prevent the sharp viscosity drop that usually occurs on heating. It should be pointed out that it is not necessary or desirable to stop gravity-driven flow. It just needs to be slowed down enough so that it does not become noticeable before the paint sets up.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail],