• Relative humidity or temperature or any combination is too high
• Solvent entrapment caused by air flow exceeding 0-5 m/sec. which can result in premature skinning
•  Incorrect viscosity (insufficient solvent)
• Spray gun held too close to surface being painted
•  Insufficient flash-off between coats
• Too much layer thickness
• Poor atomization
  

Prevention

• Adjust environment, if possible
• Add the correct amount of solvent by measure or determine by viscosity test (if applicable)
• Maintain 15-25 cm (conventional) or 35 -45 cm (airless) distance to surface
• Obey flash -off times
• Obey thickness layers
  

Remedies

• Strip and clean or sand down and repaint