C33 Identifying and Remedying Paint Failures and Defects 2 — Questions and Answers
Question 1: A newly painted exterior wall exhibits small, round craters exposing the substrate. What is the most likely cause of this defect?
- Insufficient film thickness
- Contamination by silicone or oil on the surface before painting (Correct answer)
- Using too small a roller nap
- Applying paint in direct sunlight
Correct answer: Contamination by silicone or oil on the surface before painting
Cratering (fisheyes) is caused by surface contamination from silicone, oil, grease, or wax that repels the paint film, creating round depressions.
Cratering, also called fisheyes, occurs when contaminants such as silicone, oil, grease, wax, or certain cleaning residues are present on the surface. These substances have lower surface tension than the paint, causing the wet film to pull away and form crater-like depressions. The remedy involves thoroughly cleaning the surface with a degreaser or solvent appropriate for the contaminant, possibly sanding to remove embedded contamination, and repainting.
Question 2: What causes intercoat adhesion failure between primer and topcoat on a steel surface?
- Using the same brand for both coats
- Applying topcoat before primer has fully cured or exceeding the recoat window (Correct answer)
- Using a brush instead of a roller
- Applying primer in two thin coats
Correct answer: Applying topcoat before primer has fully cured or exceeding the recoat window
Intercoat adhesion failure occurs when the topcoat is applied before the primer cures sufficiently, or after the recoat window closes and the primer surface becomes too hard for mechanical bonding.
Intercoat adhesion failure happens when the chemical or mechanical bond between coating layers fails. Applying topcoat too early traps solvents in the primer. Applying too late means the primer has fully crosslinked and the topcoat cannot chemically bond to it. On steel, this is especially common with epoxy primers that have a limited recoat window (typically 24-72 hours).
Question 3: A C-33 contractor notices white, powdery deposits forming on a freshly painted masonry wall. What is this defect called?
- Chalking
- Efflorescence (Correct answer)
- Blooming
- Saponification
Correct answer: Efflorescence
Efflorescence is the migration of soluble salts from within masonry to the surface, where they crystallize as a white powdery deposit.
Efflorescence occurs when water migrates through masonry, dissolving soluble salts and carrying them to the surface. As the water evaporates, white crystalline salt deposits remain on or beneath the paint film. Remediation requires removing the efflorescence with dry brushing or dilute acid washing, sealing the moisture source, and repainting with an alkaline-resistant primer.
Question 4: What is saponification in the context of exterior paint failure on new concrete?
- UV degradation of the paint film
- Chemical reaction between alkali in concrete and oil-based paint, breaking down the binder (Correct answer)
- Physical cracking due to substrate expansion
- Mold growth under the paint film
Correct answer: Chemical reaction between alkali in concrete and oil-based paint, breaking down the binder
Saponification is a chemical reaction where the high alkalinity of fresh concrete breaks down the oil or alkyd binder in paint, creating a sticky, soap-like film.
Saponification occurs when oil-based or alkyd paints are applied to fresh concrete or masonry with high alkalinity (pH above 10). The alkali reacts with the ester linkages in oil and alkyd resins, breaking them down into their component fatty acids and glycerol. Prevention requires allowing concrete to cure fully, testing pH before painting, and using alkali-resistant coatings.
Question 5: A customer complains about visible brush marks in the dried paint on interior trim. Which factor most contributes to this defect?
- Using a premium brush with flagged bristles
- Paint drying too quickly before it can level, often due to low humidity or excessive brushing (Correct answer)
- Applying too many coats
- Using semi-gloss instead of flat paint
Correct answer: Paint drying too quickly before it can level, often due to low humidity or excessive brushing
Brush marks remain visible when paint dries too quickly to self-level, often caused by hot/dry conditions, over-brushing, or insufficient paint on the brush.
Visible brush marks occur when the paint film sets up before it can flow and smooth out. Contributing factors include: high ambient temperature, low humidity, wind exposure, over-brushing, and insufficient paint loaded on the brush. Adding a flow and leveling conditioner extends open time and improves the finish.
Question 6: An exterior painted surface shows irregular, map-like cracking patterns that expose the substrate. What type of paint failure is this?
- Alligatoring
- Mud cracking (Correct answer)
- Checking
- Wrinkling
Correct answer: Mud cracking
Mud cracking produces deep, irregular crack patterns resembling dried mud, typically caused by applying excessively thick coats of paint or filler.
Mud cracking occurs when a coating is applied too thickly, causing the surface to dry and shrink while the interior remains wet. This differs from alligatoring (connected rectangles), checking (shallow surface cracks), and wrinkling (surface puckering). The remedy requires removing all cracked material and reapplying at the manufacturer's recommended spread rate.
C33 Identifying and Remedying Paint Failures and Defects 2
A newly painted exterior wall exhibits small, round craters exposing the substrate.
What is the most likely cause of this defect?