What causes runs or sags in paint?

• A. Applying too much paint too quickly
• B. Using the wrong reducer
• C. Too little pigment
• D. Excessive air pressure only

Answer: (A)

Runs or sags happen when the paint film becomes too heavy for the surface to support as it flows and dries. The most common reason is applying too much paint too quickly. When a thick, solvent-rich coat is laid down, the solvent can’t escape fast enough and the liquid portion doesn’t level properly. Gravity then pulls the excess paint downward, creating a sag or run.

To prevent this, use light, even coats and allow proper flash between passes, while keeping the paint’s viscosity at the right level with the reducer, and maintain correct spray technique and distance to achieve good atomization. While the reducer and drying factors affect flow and drying, they’re not the primary cause of runs. Too little pigment won’t cause a sag, and excessive air pressure mainly affects atomization and finish texture rather than directly causing runs.