Plastering over wood requires creating a suitable surface for the plaster to adhere to, as plaster doesn't stick well directly to smooth wood. One of the most effective and widely used methods involves using a metal mesh called Expanded Metal Lathing (EML).
The Best Approach: Using Expanded Metal Lathing (EML)
Based on recommended practices, one of the best ways to plaster over wood is to use Expanded Metal Lathing (EML). This method addresses the inherent issue of plaster adhesion on smooth or potentially moving wood surfaces.
What is EML?
EML is essentially a thin sheet of metal that has been cut and stretched to create a diamond-shaped mesh pattern. This pattern provides numerous keys or grips that the plaster can physically lock onto as it cures.
Why Use EML on Wood?
- Provides Texture: Wood is often too smooth for plaster to bond effectively, leading to cracking and failure. EML creates a highly textured surface that the plaster can mechanically grip.
- Accommodates Movement: Wood can expand and contract slightly with changes in temperature and humidity. EML helps to bridge potential small movements, reducing the risk of the plaster cracking.
- Cost-Effective & Easy: As highlighted in the reference, this method is generally considered cost-effective and relatively easy to apply compared to more complex framing or preparation methods.
Steps for Plastering Over Wood Using EML
Here's a simplified breakdown of the process:
- Prepare the Wood Surface: Ensure the wood is clean, dry, and free from loose paint, varnish, or debris. Address any significant structural issues or excessive movement.
- Cut the EML: Cut the Expanded Metal Lathing sheets to fit the area you need to plaster. Use appropriate snips or shears for metal mesh.
- Secure the EML: It needs to be secured into place using screws or nails. Use screws with large washers or special lathing nails designed for this purpose. Fasten the mesh securely to the wood substrate, ensuring it is taut but not overly stretched. Overlap adjacent sheets slightly (usually by about 25-50mm) and fasten through the overlap.
- Apply the Scratch Coat (Render): Apply a first layer of plaster or render, pushing it firmly into the mesh to ensure it passes through and keys onto the back of the mesh. This coat should fully embed the EML. Score the surface horizontally with a scratch comb or notched trowel while it's still wet to create a 'key' for the next coat. Allow this coat to cure sufficiently (usually 24-48 hours or more depending on conditions).
- Apply the Float Coat: Apply a second layer of plaster (the float coat) to build up the thickness and level the surface. Use a float tool to flatten and smooth the surface. This coat brings the plaster level to the desired plane. Allow this coat to cure.
- Apply the Skim Coat (Finish Coat): Apply the final thin layer of finishing plaster (the skim coat) to achieve a smooth, paintable surface. Use a trowel to apply and smooth the plaster to a high finish.
| Step | Description | Key Action |
|---|---|---|
| Preparation | Clean and prepare the wood surface. | Ensure surface is clean and stable. |
| Secure EML | Cut and attach Expanded Metal Lathing to the wood. | Use screws or nails to fasten EML securely. |
| Apply Scratch Coat | Apply first render coat, pushing it through the mesh. | Create a key by scratching the surface. |
| Apply Float Coat | Apply second coat to build thickness and level. | Flatten and smooth the surface. |
| Apply Skim Coat | Apply final thin coat for a smooth finish. | Trowel to a high, smooth finish. |
Important Considerations
- Ensure proper ventilation to allow the plaster coats to dry and cure correctly.
- Use appropriate plaster mixes designed for lathing applications if specified by the manufacturer.
- Protect surrounding areas from plaster splatter.
Using EML provides a reliable method to ensure plaster adheres effectively and durably to wood substrates, making it a preferred technique for this type of work.
