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How to: Prevent and Eliminate Runs and Sags in Paint Film

How can I fix the ridges and beads showing up in my paint finish?

You’ve painted your car, but the paint film is not adhering uniformly, and the surface now shows beads, droplets, or ridges. Runs and sags can be eliminated by removing the wet paint film, sanding, and refinishing.

In this article, I’ll answer common questions about how to prevent runs and sags and eliminate them when they happen.

  • What are runs and sags?
  • What causes runs and sags?
  • How can I repair a surface showing signs of runs and sags?
  • How do I prevent runs and sags?

What are runs and sags?

Runs and sags (also referred to as hangers, curtains, or signatures) describe localized thickening of the paint film that shows up as wavy lines or shallow, rounded ridges.

What causes runs and sags?

Errors in applying the finish are usually the cause of runs and sags. Here are seven possible causes:

  1. You over-reduced your thinner or reducer or used one that was too slow in evaporating.
  2. You applied paint materials without allowing the proper flash time between coats.
  3. You applied paint coats that were too wet because the gun was too close to the surface or the gun speed was too slow.
  4. You applied a double coat.
  5. You sprayed using an air pressure that was too low.
  6. Your spray gun was set up incorrectly or you used an unbalanced spray pattern.
  7. Your product temperature was too cold.

How can I repair a surface showing signs of runs and sags?

Repairing runs and sags can be done by:

  • Removing the wet paint film with solvent, then clean and refinish.
  • After the finish has dried, remove the excess paint by block sanding with 1200 or finer grit sandpaper, applying compound, and polishing to restore the gloss.
  • Block sand smooth and refinish.

How do I prevent runs and sags?

Use these tips to avoid runs and sags:

  • Always follow the product directions when mixing ingredients. Select the recommended solvent for the spray conditions based on temperature, humidity, air movement, and size of the repair.
  • Spray medium wet coats, allowing sufficient flash time between coats.
  • Adjust the spray gun for the best atomization and balanced spray pattern before the paint application. Hold the gun perpendicular and parallel to the panel. Adjust the speed of the pass, pattern overlap, and distance from the panel until the desired results are achieved.
  • Set the air pressure of the gun according to the product recommendations.
  • Use the recommended spray gun, including the correct fluid tip and air cap combination.
  • Allow the paint material and substrate to reach room temperature before application of the finish.

On your next paint job, use these tips to achieve a smooth new finish on your auto.