C33 Surface Preparation for Various Substrates 4 — Questions and Answers
Question 1: When preparing a previously painted concrete floor that shows efflorescence, what is the correct treatment before applying a new coating?
- Apply primer directly over the efflorescence
- Neutralize with a dilute acid wash and rinse thoroughly (Correct answer)
- Sand the surface smooth and coat immediately
- Apply two coats of latex paint to seal the salts
Correct answer: Neutralize with a dilute acid wash and rinse thoroughly
Efflorescence (salt deposits) on concrete must be removed with a dilute acid wash (such as muriatic acid) and thoroughly rinsed before any coating is applied.
Question 2: A painting contractor is prepping galvanized metal that has never been painted. Which primer type is most appropriate for this substrate?
- Oil-based alkyd primer
- Zinc-rich self-etching primer (Correct answer)
- Shellac-based primer
- Standard latex drywall primer
Correct answer: Zinc-rich self-etching primer
New galvanized metal requires a zinc-rich or self-etching primer to promote adhesion, as its zinc coating resists bonding with conventional primers.
Question 3: What minimum ambient temperature is generally recommended when applying water-based (latex) coatings to exterior wood substrates?
- 25°F (-4°C)
- 35°F (2°C)
- 50°F (10°C) (Correct answer)
- 65°F (18°C)
Correct answer: 50°F (10°C)
Most latex coatings require a minimum ambient and surface temperature of 50°F (10°C) for proper film formation and adhesion.
Question 4: When pressure washing a wood substrate prior to painting, what is the recommended drying time before applying paint in typical California conditions?
- 2 hours
- 6 hours
- 24–48 hours (Correct answer)
- 72 hours minimum
Correct answer: 24–48 hours
Wood should be allowed to dry at least 24–48 hours after pressure washing to ensure moisture content is low enough for paint adhesion.
Question 5: A contractor encounters a stucco surface with existing alligator cracking. What is the correct preparatory step?
- Apply a high-build elastomeric coating directly to bridge the cracks
- Grind out cracks, fill with appropriate patching compound, and allow to cure (Correct answer)
- Sand the surface and spot prime cracked areas only
- Wet the surface and apply paint while still damp to improve penetration
Correct answer: Grind out cracks, fill with appropriate patching compound, and allow to cure
Alligator cracks in stucco must be ground out and filled with a compatible patching compound before any coating is applied to ensure a stable substrate.
Question 6: Which condition on a steel substrate requires immediate attention before blast cleaning to avoid creating a hazardous situation?
- Surface rust (mill scale)
- Soluble salts (chlorides)
- Oil or grease contamination (Correct answer)
- Tight, adherent old paint
Correct answer: Oil or grease contamination
Oil or grease must be removed by solvent cleaning before blast cleaning, as abrasive blasting drives contaminants into the surface rather than removing them.
Question 7: When preparing interior drywall in new construction for a flat latex finish, what surface profile is typically required?
- A roughened 2-mil profile created by sanding
- A smooth, well-primed surface free of tool marks and imperfections (Correct answer)
- A lightly scuffed surface with 80-grit sandpaper only
- No surface preparation — paint can be applied directly to bare drywall
Correct answer: A smooth, well-primed surface free of tool marks and imperfections
Interior drywall for flat latex finishes requires a smooth, primed surface free of defects, tool marks, and fastener pops so that the finish coat lies uniformly.
C33 Surface Preparation for Various Substrates 4
When preparing a previously painted concrete floor that shows efflorescence, what is the correct treatment before applying a new coating?