To successfully plaster over painted plaster, the key lies in thorough preparation of the existing surface to ensure proper adhesion of the new plaster.
Essential Steps for Plastering Over Painted Plaster
Plastering over painted surfaces requires more than just applying a new coat; it demands careful preparation to create a suitable "key" for the plaster to bond effectively. Without proper preparation, the new plaster may crack, peel, or de-laminate over time.
Here are the crucial steps involved:
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Surface Preparation: Scraping and Cleaning
- Remove Loose Paint: Begin by meticulously scraping the entire painted surface. This step is critical to remove any loose, flaking, or peeling paint. A robust scraper or wire brush can be used for this task. The goal is to achieve a stable, sound base.
- Clean the Surface: After scraping, clean the wall thoroughly to remove dust, debris, and any grease or dirt. A damp cloth or sponge can be used, ensuring the surface is dry before proceeding.
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Creating a "Key": Scratching the Surface
- Enhance Adhesion: Once the surface is clean and free of loose paint, the next vital step is to create a mechanical key. This involves scratching the painted plaster with wavy lines.
- Why Wavy Lines? As one expert highlights, "cracks always travel in a straight line." By creating wavy or irregular scratches, you disrupt potential straight-line weaknesses, providing numerous anchor points for the new plaster. This significantly improves the bond and reduces the risk of the plaster cracking or detaching. Use a plasterer's scratcher tool or even a stiff wire brush to score the surface deeply enough to expose the underlying plaster in places.
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Priming the Surface
- Bonding Agent Application: After scraping and keying, the surface must be primed to ensure the new plaster adheres correctly and dries at an even rate. This prevents the old painted surface from "sucking" too much moisture out of the new plaster too quickly, which can lead to cracking.
- Primer Options:
- PVA (Polyvinyl Acetate): Apply two coats of PVA. The first coat should be applied the night before plastering to allow it to dry completely. The second coat should be applied shortly before plastering, ideally when it's still tacky, to provide a strong bond.
- Specialized Plasterers' Primer: Alternatively, use a purpose-designed plasterers' primer such as Thistle Bond-It. These primers are formulated to create an excellent bond on difficult surfaces and often contain aggregate to provide an even stronger mechanical key. Apply one coat according to the manufacturer's instructions.
Materials and Tools
To successfully plaster over painted plaster, you will need specific tools and materials for preparation:
| Category | Item | Purpose |
|---|---|---|
| Preparation | Scraper / Wire Brush | Removing loose paint and debris |
| Scratching Tool | Creating a "key" for plaster adhesion | |
| Cleaning Cloth / Sponge | Cleaning dust and dirt from the surface | |
| Priming | PVA (Polyvinyl Acetate) | Standard bonding agent, applied in two coats |
| Plasterers' Primer | Specialized bonding agent (e.g., Thistle Bond-It) | |
| Paint Brush / Roller | Applying the primer | |
| Safety | Dust Mask, Safety Goggles | Protection during scraping and cleaning |
By diligently following these preparation steps—scraping away loose paint, scratching the surface for a mechanical key, and applying an appropriate primer—you can create an ideal foundation for a durable and smooth new plaster finish.
