• Dirty air cap and fluid tip (distorted spray pattern)
• Gun stroked too close to the surface
• Trigger not released at end of stroke (when stroke does not go beyond object)
• Gun stroked at wrong angle to surface
• Coating piled on too heavy
• Coating material thinned out too much
• Incorrect viscosity (too much solvent)
• High film build on overlaps
• Incorrect fluid tip, needle or air cap (poor atomization)
• Slow application resulting in high film build
• Poor routing resulting in irregular layer thickness
•  Insufficient flash-off between coats
  

Prevention

• Remove air cap and clean tip and air cap carefully
• Maintain 15-25 cm (conventional) or 35 –45 cm (airless) distance to surface
• Release the trigger after every stroke
• Keep gun at right angle (perpendicular) to surface during stroke
• Develop ability to apply correct thickness by panel practice
• Add the correct amount of solvent by measure or determine by viscosity test
  

Remedies

• Runs that have already dried can be removed by sanding/polishing and repainting