Plastering over painted walls requires careful preparation to ensure the new plaster adheres properly. Based on expert advice, the process involves several key steps before applying the plaster itself.
To plaster over painted walls, you must first prepare the surface by scraping loose paint, addressing cracks, ensuring the underlying plaster is sound, and applying a bonding agent like PVA. Only then can you apply the new plaster.
Essential Steps for Plastering Painted Walls
Successfully plastering over a painted surface hinges on proper preparation. The paint creates a barrier that prevents the new plaster from bonding directly to the wall substrate. A bonding agent is crucial, but surface defects must be handled first.
Here is a breakdown of the process, incorporating the critical steps recommended by professionals:
1. Surface Preparation
Before even thinking about mixing plaster, the wall needs thorough preparation. This stage is non-negotiable for a lasting finish.
- Scrape Loose Paint: Start by removing any paint that is flaking, bubbling, or peeling. Use a scraper to get under the edges and lift it away. It's crucial to scrape back any flakey bits of paint.
- Brush Away Dust: Once loose paint is removed, brush the entire wall surface down to get rid of dust and debris. Brush away dust to ensure a clean surface for the bonding agent.
- Check for Blown Plaster: Tap the wall surface lightly with your knuckles or a tool handle. If you hear a hollow sound, the underlying plaster may have separated from the wall (known as "blown" plaster). You must make sure the plaster hasn't blown in any places. Blown plaster needs to be removed and repaired before proceeding.
- Address Cracks: Minor cracks, often called air line cracks, should be dealt with. Scrim any air line cracks using a self-adhesive fiberglass scrim tape. This helps prevent them from reappearing in the new plaster.
2. Apply Bonding Agent (PVA)
With the surface clean, stable, and prepared, the next step is applying a bonding agent. Polyvinyl Acetate (PVA) is commonly used for this purpose as it seals the surface and provides a 'key' for the new plaster to stick to.
- Give the walls two coats of PVA. Dilute the PVA according to the manufacturer's instructions for the first coat (often 1 part PVA to 5 parts water). Apply evenly.
- Apply a second coat, often at a stronger dilution (e.g., 1 part PVA to 3 parts water) or even undiluted, depending on the surface porosity and product instructions. Follow it up with 2 coats of Pva. Ensure the first coat is touch-dry before applying the second.
3. Apply Plaster
Once the PVA has dried sufficiently (it should feel tacky or firm, depending on the specific product and conditions), you can begin applying the plaster.
- When dried apply two coats plaster. Apply the first coat of plaster evenly across the wall using a plastering trowel. This coat fills imperfections and provides the base.
- Allow the first coat to firm up but not fully dry. Apply the second, finishing coat of plaster. This coat is smoothed to achieve the final flat surface ready for painting or decorating.
Following these steps, focusing on thorough preparation and the correct application of a bonding agent like PVA, is essential for achieving a durable and smooth plastered finish over a painted wall.
