CSLB C-33 Painting & DecoratingCoating ApplicationMedium
A painting contractor is applying a clear lacquer finish to custom-built wood cabinetry. After the first coat, the surface appears dull and hazy, lacking the expected clarity and gloss. This phenomenon is often referred to as 'blushing' or 'whitening'. What is the most common environmental factor contributing to this defect?
- AHigh relative humidity.
- BLow relative humidity.
- CExcessively high ambient temperature.
- DInsufficient ventilation in the spray booth.
Show answer & explanationAnswer & explanation
Correct answer: A. High relative humidity.
Blushing (or whitening) in lacquer finishes is primarily caused by high relative humidity. Rapid evaporation of solvents cools the surface, causing moisture in the air to condense on the wet film. This trapped moisture, combined with certain resin components, creates a cloudy or hazy appearance.
Why the other options are wrong
- B. Low humidity is ideal for lacquer application, as it prevents moisture condensation and blushing.
- C. High temperatures would generally accelerate drying, potentially leading to other issues like dry spray, but not typically blushing.
- D. Insufficient ventilation can lead to solvent buildup and slow drying, but blushing is specifically tied to humidity and temperature effects on solvent evaporation.
Blushing (Lacquer)
A defect in lacquer finishes characterized by a milky, cloudy, or hazy appearance, typically caused by the condensation of moisture onto the rapidly cooling wet film due to fast solvent evaporation in high humidity.
- Caused by high humidity and/or rapid solvent evaporation.
- Trapped moisture in the film scatters light, causing dullness.
- Can sometimes be remedied by applying a 'blush retarder' or 'flow-out' solvent.
Memory trick: High humidity makes lacquer blush, causing a hazy rush.