Unless you know what you are doing, plastering external corners (such as a chimney breast) can be difficult. The problem occurs when trying to create a strong, straight corner. If you do not use mesh angle beading on the corner, this will be almost impossible and any knocks will quickly break the plaster away. You can buy angle bead cheaply from any DIY store or builders merchant.
Step 1 – Make sure that the blocks or bricks forming the corner are clean and free from dirt and debris. Apply some small dabs of plaster onto the wall on both faces of the corner. The dabs should be spaced around 300mm apart, all the way up the wall. These will act as an adhesive to hold the mesh angle bead in place at the corner of the wall.
Step 2 – Measure the height of the wall from floor to ceiling and cut your length of mesh angle bead to the correct size. Press the bead into the still wet dabs of plaster, making sure that it is straight and even the whole way up the wall. The plaster dabs will squeeze through the mesh and, when it sets, hold it firmly in place.
Step 3 – Let the mesh angle bead set firmly in place before you start to apply the undercoat plaster to the rest of the wall. When you reach the corner, apply the plaster level with the bead. As the undercoat plaster begins to go off, scrap it back next to the bead by a couple of millimetres so that the surface coat can be finished flush with the edge when it is applied. If you don’t do this, the surface coat will stand proud of the bead and result in a less than perfect finish.
When you apply the surface coat, just the very corner of the bead should be visible. This will easily be covered by paint or wallpaper.