CSLB C-33 Painting & DecoratingCoating ApplicationEasy
A painting contractor is applying a stain-blocking primer to water-damaged drywall before painting. The primer is an oil-based alkyd. After application, the contractor notices that the primer is taking an unusually long time to dry to the touch, despite good ventilation. Which of the following is the most likely cause?
- AHigh relative humidity is accelerating the drying process.
- BThe ambient temperature is too low for optimal alkyd curing.
- CThe primer was applied too thinly, preventing proper film formation.
- DThe primer was over-thinned with a fast-evaporating solvent.
Show answer & explanationAnswer & explanation
Correct answer: B. The ambient temperature is too low for optimal alkyd curing.
Oil-based (alkyd) primers and paints cure through oxidation, a chemical reaction that is significantly slowed down by low temperatures. If the temperature is too low, the drying time will be extended considerably.
Why the other options are wrong
- A. High relative humidity can slow the drying of water-based paints but has less direct impact on the chemical cure of oil-based alkyds, and certainly wouldn't accelerate it.
- C. Applying too thinly would generally lead to faster drying, but poor coverage, not extended drying time.
- D. Over-thinning with a fast-evaporating solvent would likely lead to faster initial solvent flash-off, not necessarily slower overall cure for an alkyd.
Alkyd Paint Curing
Alkyd paints cure primarily through oxidation, a chemical reaction with oxygen in the air, which is influenced by temperature and humidity.
- Requires oxygen for curing.
- Lower temperatures slow the oxidation process.
- High humidity can slightly retard, but temperature is a dominant factor.
Memory trick: Alkyd's slow, temperature's low, that's the flow.