The scratch coat is an essential process in preparing surfaces for both external and internal walls before plastering begins. The goal of the scratch coat is to provide a rough layer that allows for better adhesion of the final plaster layer, enhancing the strength of the bond and ensuring the plaster does not crack or fall off over time. Bond is added to the scratch coat mixture as an adhesive agent to improve the mortar’s adhesion to concrete or brick surfaces.
1. Recommended ratios for the scratch coat and bond mixture
- For the scratch coat, a mixture of sand and cement is typically used at a ratio of:
- 1:3 (one part cement and three parts sand).
- Bond is added to improve adhesion between the scratch coat and the wall.
- The recommended ratio for adding bond is 1 to 2 liters of bond per 50 liters of water in the mixture.
- For a large drum (around 200 liters of water), 5 to 7 liters of bond should be added to ensure good adhesion.
2. When to begin the scratch coat after construction work?
Once the walls have been built, the wall must be given enough time to dry and settle before the scratch coat is applied. The time period depends on weather and site conditions, but the general rule is to wait:
- 7 to 10 days after the wall construction is completed.
- This period allows the walls to settle and release excess moisture, preventing expansion or contraction issues that could cause cracks in the scratch coat or plaster.
3. Importance of curing the walls after the scratch coat
After applying the scratch coat, the surface must be cured properly to ensure cohesion and durability. Curing involves:
- Regularly spraying water for at least 3 days after the scratch coat. This process ensures the scratch coat dries slowly, preventing cracks and enhancing mortar adhesion to the walls.
- Spraying prevents the mortar from drying too quickly and reduces the risk of surface cracks.
4. Detailed mix ratios
To create an ideal scratch coat mixture, the following model can be used:
| Component | Ratio/Volume |
|---|---|
| Cement | 1 part |
| Sand | 3 parts |
| Water | 50 liters per 200 liters |
| Bond | 5-7 liters per 200 liters |
5. Benefits of using bond in the scratch coat
- Enhanced adhesion: Bond acts as a strong adhesive agent between the wall and the scratch coat mixture, ensuring durable and long-lasting plaster adhesion.
- Reduction of cracks: Bond helps reduce the occurrence of cracks caused by the expansion and contraction of walls over time, especially in areas exposed to temperature or humidity fluctuations.
- Improved water resistance: Adding bond to the scratch coat makes the surface more resistant to water infiltration, which is crucial for exterior areas or wet spaces like bathrooms.
6. Key notes when using bond
- Ensure the surface is clean and free of dust before starting the scratch coat.
- When adding bond, it should be well-mixed with water before being added to the sand-cement mixture to ensure even distribution.
- Avoid quick drying: The scratch coat requires careful curing, and water should be sprayed regularly to maintain moisture and prevent cracks.
Conclusion:
- The scratch coat is a critical step in preparing walls for plastering, and attention to detail is necessary, such as using bond to ensure strong adhesion and durability.
- Waiting 7 to 10 days after construction before starting the scratch coat is essential to ensure wall stability.
- Treating the walls with water for at least 3 days after the scratch coat is crucial to ensure proper drying and cohesion.
Photo credit: Wedorender
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