Before plastering, the surface has to be:
🔬 Understanding the Principle of Plaster Adhesion
For plaster to adhere strongly to a wall, it needs to form a powerful bond with the underlying surface (the substrate). This bond is primarily a mechanical bond, meaning the plaster physically interlocks with the surface it's applied to. The question asks for the ideal surface condition to achieve this.
Mechanical Key: This refers to the texture of a surface that provides a "key" or grip for a new layer of material. A surface with a good mechanical key has a high degree of roughness, with many small indentations and protrusions that the plaster can flow into and lock onto as it hardens.
⚖️ Detailed Analysis of the Options
Let's evaluate each surface condition to determine which is best for plaster adhesion.
(a) Rough
Why it's correct: A rough surface provides the best possible mechanical key. The increased surface area and the numerous crevices allow the mortar to grip the wall firmly, creating a strong, durable bond. For new brickwork, this is achieved by "raking" the mortar joints (scraping them out to a depth of about 10-15 mm). For old or smooth concrete surfaces, the surface is often intentionally roughened by hacking or sandblasting before plastering.
(b) Smooth
Why it's incorrect: A smooth, glossy surface offers a very poor mechanical key. The plaster has nothing to grip onto and is highly likely to lose its bond, leading to delamination or peeling. This is why smooth concrete surfaces must be roughened before plastering.
(c) Cemented
Why it's not the best answer: This describes the *material* of the surface (e.g., a brick wall with cement mortar joints), not its required *condition*. While the wall may be "cemented," its surface texture must still be made rough to ensure a good bond.
(d) Watered
Why it's a step, but not the answer: Watering (or curing) the surface is a critical step in preparation, but it serves a different purpose. It prevents the dry, absorbent wall from sucking the water out of the fresh plaster too quickly. If this happens, the cement won't have enough water to hydrate properly, resulting in a weak, crumbly plaster. However, this is done *after* ensuring the surface has the correct rough texture.
💡 Study Tips for Plastering
- Rough = Grip, Smooth = Slip: This is the most fundamental principle of adhesion for plaster and many other coatings.
- Remember the Two "R's": Proper plaster preparation involves making the surface Rough and then making it Ready (by watering/damping). The question asks about the texture.
- Think of Raking Joints: The common practice of raking out mortar joints in new brickwork is a perfect example of intentionally creating a rough surface to provide a key for the plaster.