Which of the following defects occurs when the surface to be painted is too smooth?
🔬 Understanding Surface Adhesion and "Key"
For paint to adhere properly, it needs a surface with a "key" or "tooth." This refers to a microscopic level of roughness that gives the liquid paint something to grip onto. If a surface is too smooth, glossy, or non-porous, the paint struggles to form a strong mechanical bond.
Running: This is the specific paint defect that occurs when wet paint fails to adhere to a very smooth surface. The paint pulls back on itself, creating streaks and leaving small, uncovered patches. It's a direct failure of the initial bond between the paint and the substrate.
Why a Smooth Surface is a Problem:
Without a proper key, the surface tension of the liquid paint can be stronger than its adhesive force to the smooth surface. This causes the paint to bead up or pull away from certain areas, much like water on a waxed car. This is why sanding glossy surfaces before repainting is a critical preparation step.
⚖️ Detailed Guide to Other Paint Defects
It's important to distinguish running from other defects, especially sagging, which can look similar but has a different cause.
(b) Running
Appearance: Paint pulls away from areas, leaving streaks and bare spots.
Core Issue: Poor adhesion due to a surface that is too smooth or glossy.
(a) Sagging
Appearance: Droopy, curtain-like drips on vertical surfaces.
Core Issue: Paint applied too thickly. Gravity pulls the excess weight of the wet paint down before it can set. The surface itself might have a perfect key, but the paint layer is simply too heavy.
(c) Grinning
Appearance: The underlying surface is visible through the topcoat.
Core Issue: Insufficient opacity (hiding power) of the paint. This is a problem with the paint's formulation or thin application, not the surface texture.
(d) Wrinkling
Appearance: A rough, crinkled surface.
Core Issue: The top layer of paint dries faster than the bottom layer, typically from being applied too thickly. This is a drying issue, not a surface adhesion issue.
💡 Study Tips for Paint Defects
- Running Runs Away from Smoothness: The paint can't "grip" the smooth surface, so it "runs" away.
- Sagging is from Self-Weight: Sagging is caused by the paint being too heavy, not the surface being too smooth. Think of a heavy bag "sagging."
- Running vs. Sagging: The key difference is the cause. Running = Smooth Surface. Sagging = Thick Application.
- Grinning is See-Through: The old surface is "grinning" or peeking through the new coat.