Where plaster has broken off of an external corner (such as the corner of a chimney breast) it can be difficult to make a good repair. Making a sharp corner and achieving a level finish is harder because you often have no solid surface to press the wet plaster against. The answer to this is to fix a wooden batten to the wall to give you that solid surface.
The first thing you should do is carefully chip back the edges of the broken section of plaster until the edge is solid and not crumbling. Now use a dry brush to clear away any dust and loose plaster from the hole. To make your batten, select a piece of straight wood which is at least 400mm longer than the broken section of plaster and around 60mm wide. Drive two masonry nails almost through the wood at each end, slightly closer to one side than the other. When you hammer this into the wall, this should help to prevent any more plaster breaking away.
Hold the batten up against the broken section with the nails furthest away from the corner. Hold a spirit level or straight edge against the adjacent wall and use it to ensure the batten is aligned with the surface of the plaster at the top and bottom of the broken section. If you bear in mind that you are trying to create a false edge to the broken section so that the plaster applied along one wall will have something to be pressed against, this process should make more sense. Nail the batten carefully into place by driving the nails into the wall above and below the broken section, but leaving the heads of the nails protruding from the wood.
Now you have a false edge to the broken corner, you can start to fill the hole with either one-coat or undercoat plaster. Build the plaster up until it is flush with the wall on one side and the batten on the other before smoothing the surface. Allow the plaster to dry for the recommended time and then carefully pull the nails out of the batten and remove it from the wall. You can now repeat the process on the other side of the broken corner, being extra careful not to dislodge the new plaster as you attach the batten.
The broken section should now be filled nicely and you should once again have a fairly smooth and sharp corner. With a little bit of careful sanding, the corner shoould look as good as new. However, if you want to go that extra step you can finish the undercoat plaster just below the surface of the wall on both sides of the corner and then repeat the whole process to apply a surface coat of plaster. This can then either be smoothed off with a corner trowel to create a smooth, even edge, or you can carefully blunt the corner with a normal (slighty wet) trowel.
Reinforcing A Corner
If the corner which needs repairing is prone to damage, or if the amount of plaster broken off of it is very large, you might want to consider reinforcing it with expanded metal mesh beading. This is made up of two strips of galvanised steel mesh, set at right angles along a rounded central strip. The rounded central strip forms the corner, while the mesh binds it into the surrounding plaster. This is sold in long lengths, but can be cut easily with tin snips and a hacksaw. The cut edge should be treated with metal primer before being used.
Cut the mesh beading to the required length and attach it to the wall in the broken section by applying small dabs of undercoat plaster and pressing the mesh into them. Use a spirit level and a straight edge to ensure that the rounded central strip is level with the surface of the plaster along both walls. Once you have it in place, press it gently into the dabs of plaster so that the metal bead will not show above the finshed plaster. Allow this to dry in place and then you can use it as a guide to creating a straight edge down the corner of the wall instead of using a wooden batten.