Startseite/Home-English/Paint defects/Sags and runner
Läufer durch Überbeschichtung runner sag
tropfen in stehender fläche
Klarer Läufer clearcoat sag
Läufer

Sags and runner

  • Sources of error:
    • The distance to the object is too short.
    • The material viscosity is too low (material is thin).
    • The dilution and/or hardener selection is too long or was not adapted to the component size or temperature.
    • The material was applied too thickly.
    • The specified flash-off times were not adhered to (waiting too short causes the material to fall below the chemical bonding time and can „liquefy“ further.)
    • Nozzle size of the gun too large. This must be adapted to the material parameters. For low viscosity materials (water) it will be smaller and for high viscosity materials (honey) it will be larger.
    • The object surface and/or material too cold. Here, the temperature difference can cause an „interference layer“, condensation layer, and prevents the material from the hoped-for adhesion.
    • Material diluted higher because of thick liquid.
    • The drawing speed of the painting cycle was too slow. Here the gun must be pulled a little faster.
  • Error prevention:
    • Change spraying technique.
    • Adjust material according to technical information and do not „over-thin“ on your own authority.
    • Check gun and nozzle equipment.
    • Follow the application instructions in the material manufacturer’s technical information.
  • Repair procedure:
    • Allow runner to dry thoroughly. If necessary, dry with infrared. Runners tend to be hard on the outside and not thoroughly dried.
    • Remove and polish up via the finish path. If there is too much curtaining, the result is more likely to be achieved with a new coat of paint.
  • What happens in this process?
    The material submits to gravity. The tensile forces are higher than the holding force on the object (adhesion) and in the material itself (cohesion). However, the layers do not separate but redistribute themselves on the surface.
    Rearing and „pushing“ in the material like a wave on the sea. At break-even, the only way is down.
    Solvents open the previous material passage. If the flash-off time was too short, the material does not „lay down“ on top of it but bonds internally and becomes too thick, and starts to run.

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