Fixing damaged outside corners of plaster walls involves two primary methods, depending on the extent of the damage: you can repair minor damage with undercoat plaster using a temporary board to achieve a neat edge, or for longer, more extensive damage, it's more effective to reinforce the corner with metal or plastic corner beading. Both approaches aim to restore the corner's structural integrity and aesthetic appeal.
Understanding the Damage
Before beginning any repair, it's crucial to assess the extent and type of damage. Minor chips or cracks on a small section of the corner might only require a simple plaster patch. However, if a significant length of the corner is crumbling, severely cracked, or completely broken off, reinforcing with corner beading will provide a more durable and long-lasting solution.
Method 1: Repairing Small Damage with Undercoat Plaster
This method is ideal for smaller chips, dings, or areas where the plaster has flaked off but the underlying structure is still sound. It relies on creating a temporary guide to achieve a perfectly straight and sharp edge.
Materials & Tools Needed:
- Undercoat Plaster: Specifically designed for initial layers and building up thickness.
- Mixing Bucket & Trowel/Hawk: For preparing and applying plaster.
- Straight Edge/Board: A piece of wood or metal, ideally 4-6 inches wide and longer than the damaged area, to act as a guide.
- Small Nails/Screws & Hammer/Drill: To temporarily secure the board.
- Spirit Level: To ensure the board is plumb.
- Water: For mixing plaster.
- Sponge & Bucket: For cleaning.
- Safety Glasses & Gloves: For personal protection.
Step-by-Step Repair Process:
- Preparation:
- Carefully remove all loose or crumbling plaster from the damaged area. Use a scraper or a stiff brush.
- Clean the exposed surface thoroughly to remove dust and debris. Dampen the area slightly if it's very dry to prevent the plaster from drying too quickly.
- Attach the Guide Board:
- Position your straight edge or board along one side of the damaged corner. Ensure it runs perfectly plumb (vertical) using a spirit level.
- Temporarily nail or screw the board into the wall at several points, making sure it's firmly secured and won't move during plaster application. The board will act as a formwork for your new corner edge.
- Apply Undercoat Plaster:
- Mix the undercoat plaster according to the manufacturer's instructions. Aim for a consistent, workable consistency.
- Using your trowel or hawk, apply the plaster to the damaged area, pressing it firmly into the void. Build up the plaster until it's slightly proud of the board's edge.
- Smooth the plaster against the guide board, using the board as a straight edge to create a crisp, new corner.
- Allow to Set:
- Let the plaster begin to set (firm up) according to the product's guidelines. This usually takes 30-60 minutes, depending on the mix and environmental conditions.
- Remove Board & Finish:
- Carefully remove the temporary guide board once the plaster is firm enough to hold its shape but not fully hardened.
- Using your trowel or a damp sponge, gently neaten any imperfections on the newly formed edge. Allow the plaster to dry completely.
- Once dry, you may apply a finishing plaster layer (skim coat) if necessary and then sand smooth before priming and painting.
Method 2: Reinforcing with Metal or Plastic Corner Beading
This method is highly recommended for longer stretches of damaged corners or in areas prone to repeated impact, as it provides a robust, reinforced edge. Corner beads (often called angle beads) are pre-formed profiles that offer excellent protection and a perfectly straight line.
Materials & Tools Needed:
- Metal or Plastic Corner Beading: Choose a size appropriate for your wall thickness. Metal beads (galvanized or stainless steel) are very strong, while plastic beads are rust-proof and lighter.
- Snips/Hacksaw: For cutting the beading.
- Plasterboard Adhesive/Nails/Screws: For fixing the bead to the wall. Some beads are fixed with plaster dabs.
- Trowel/Hawk: For applying plaster.
- Undercoat and Finishing Plaster: To embed and cover the bead.
- Spirit Level: To ensure the bead is plumb.
- Tape Measure & Pencil: For accurate measurement.
- Safety Glasses & Gloves: For personal protection.
Step-by-Step Reinforcement Process:
- Preparation:
- Remove all loose and crumbling plaster from the damaged corner. Ensure the wall surface is clean and relatively even.
- If the existing corner is severely recessed, you might need to build up the wall slightly with a filler or undercoat plaster before applying the bead.
- Measure and Cut Beading:
- Measure the exact length of the damaged corner you need to cover.
- Using snips for plastic or a hacksaw for metal, cut the corner bead to the precise length.
- Fix the Corner Beading:
- Apply dabs of plasterboard adhesive or rapid-set plaster mix along the inside flanges of the bead, or directly to the wall where the bead will sit. Alternatively, some beads are designed to be nailed or screwed into the wall studs if accessible.
- Press the corner bead firmly into place, ensuring it is perfectly plumb with a spirit level. The nose of the bead should be the proudest part, ready to form the new corner.
- Check for straightness and adjust as needed. Allow the adhesive or plaster dabs to set sufficiently to hold the bead firmly.
- Apply Plaster Over Beading:
- Once the bead is securely fixed, apply an undercoat plaster to both sides of the corner, working from the bead outwards. The bead acts as a guide, allowing you to achieve a perfectly straight and sharp corner.
- Build up the plaster layers gradually, ensuring the plaster meets the nose of the bead without covering it entirely.
- Allow the undercoat to set.
- Finishing Plaster:
- Apply a final skim coat of finishing plaster over the undercoat and the bead, ensuring a smooth, seamless finish. The aim is for the plaster to be flush with the nose of the bead, creating a sharp, durable edge.
- Smooth the plaster with a clean trowel.
- Drying and Sanding:
- Allow the plaster to dry completely, which can take 24-48 hours depending on humidity and plaster thickness.
- Lightly sand any rough spots or imperfections with fine-grit sandpaper for a perfectly smooth surface, ready for priming and painting.
By following these methods, you can effectively repair and reinforce outside plaster wall corners, restoring their integrity and appearance.
