CSLB C-33 Painting & DecoratingSurface PreparationHard
A painting contractor is preparing an interior plaster wall that has several areas of peeling paint. The existing paint is a non-alkyd, water-based paint. What is the most important test to perform after removing the loose paint but before applying new primer?
- AAn adhesion scratch test.
- BA lead paint test.
- CA pH test of the plaster surface.
- DA moisture meter test.
Show answer & explanationAnswer & explanation
Correct answer: C. A pH test of the plaster surface.
New or improperly cured plaster can be highly alkaline (high pH), which can cause saponification and failure of many water-based and some oil-based paints. A pH test will determine if the plaster is sufficiently neutral for painting. Moisture testing is important but doesn't reveal alkalinity. Lead paint testing is for safety but not adhesion. Adhesion scratch tests are for existing paint, not the underlying substrate after removal.
Why the other options are wrong
- A. An adhesion scratch test is typically performed on existing, intact coatings to assess their bond strength, not on a bare plaster substrate.
- B. A lead paint test is crucial for safety on older homes but does not address adhesion issues related to plaster chemistry.
- D. A moisture meter test is important for ensuring the wall is dry, but it will not detect alkalinity, which can cause paint failure on plaster.
Plaster pH Testing
Before painting new or repaired plaster, a pH test is essential to ensure the surface is sufficiently neutral (below 9) to prevent paint failure due to alkalinity.
- High pH (alkalinity) causes saponification.
- Saponification leads to paint degradation.
- Alkali-resistant primers mitigate high pH.
Memory trick: Peeling Plaster's pH must be Perfected for Paint's Proper bond.