Plastering is applied to a wall surface in a multi-step process involving mixing the plaster, applying base coats for strength and leveling, and then applying finish coats for a smooth and aesthetically pleasing surface.
Here's a breakdown of the typical plastering application process:
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Surface Preparation: This is a crucial step.
- Cleaning: Ensure the wall surface is clean, free from dust, dirt, grease, and loose particles. Remove any old wallpaper or flaking paint.
- Keying: If the existing surface is very smooth (like glazed tiles), create a "key" for the plaster to adhere to. This can involve etching the surface or applying a bonding agent.
- Priming: Apply a suitable primer to the wall. This helps with adhesion, prevents excessive suction from the wall (which can cause the plaster to dry too quickly and crack), and can help to seal the surface.
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Mixing the Plaster:
- Ratio: Follow the manufacturer's instructions for the correct plaster-to-water ratio. Typically, plaster powder is added to clean water gradually to prevent lumps.
- Consistency: Mix thoroughly until you achieve a smooth, creamy paste with no lumps. The consistency should be similar to thick paint or yogurt.
- Working Time: Plaster has a limited working time, so only mix as much as you can apply within that timeframe (typically 30-60 minutes, depending on the plaster type).
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Applying the Base Coat (Scratch Coat):
- Application: Use a hawk and trowel to apply the first coat of plaster. Load the trowel from the hawk and apply the plaster to the wall with a firm, even pressure. Aim for a thickness of around 6-10 mm.
- Coverage: Ensure the entire wall surface is covered with the base coat.
- Scratching: Once the base coat has partially set (but is still soft), scratch the surface with a notched trowel or comb. This creates a mechanical key for the next coat to adhere to. This coat is called the "scratch coat" for this reason.
- Curing: Allow the base coat to dry completely before applying the next coat. This typically takes 24-48 hours, depending on the environment.
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Applying the Second Base Coat (Float Coat - Optional):
- Purpose: If the wall surface is very uneven, a second base coat may be necessary to achieve a level surface. This coat is called the "float coat".
- Application: Apply the float coat in a similar manner to the scratch coat, ensuring even coverage.
- Floating: After the float coat has partially set, use a damp sponge or float to level and smooth the surface.
- Curing: Allow the float coat to dry completely.
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Applying the Finish Coat (Top Coat or Skim Coat):
- Application: This is the final coat that provides the smooth, aesthetic finish. Apply a thin, even layer of finish plaster (typically 2-3 mm thick).
- Technique: Use a clean trowel and apply the plaster with overlapping strokes, using firm pressure to compact the plaster and remove any air bubbles.
- Smoothing: As the plaster begins to set, use a clean, damp trowel to smooth the surface to the desired finish. This may involve several passes with the trowel.
- Polishing (Optional): Some plasterers polish the surface to create a very smooth, almost glossy finish.
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Curing and Drying:
- Drying Time: Allow the plaster to dry slowly and evenly. Avoid rapid drying, which can cause cracking.
- Ventilation: Ensure adequate ventilation to promote drying.
- Protection: Protect the plastered surface from damage during the drying process.
In summary, plastering involves careful surface preparation, precise mixing of materials, and a multi-layered application technique using base coats for strength and leveling, and a finish coat for a smooth and aesthetically pleasing surface.
