When repairing lath and plaster walls, it is highly advisable to get into the habit of using similar materials. Repairing holes in lath and plaster walls can range from being very difficult to quite quick and simple. If laths themselves are damaged, an entire section of the wall may need replacing.
Step1 - How to Repair Small Holes in the Plasterwork
Repairing small holes in plasterwork is much simpler than replacing laths or fixing major holes in the wall. First, you should be sure to clean the surface from any loose plaster. To prepare for applying the filler, try to undercut the plasterwork at the edges so that there is a good grip available for the new filler. Mix the plaster filler as directed to by the instructions that come with it. Using a sponge, dampen the sides of the plasterwork inside the hole. You can then apply the filler to the hole, making sure that it is thoroughly level. To make it as level as possible, use the scraper held at a 45-degree angle over the surface to smoothen it out. You may also need to apply a second coat. When each application of the filler is dry, you should gently rub it down with sandpaper to get a completely smooth surface. This will help to ensure that the repaired area will blend in seamlessly with the rest of the wall.
Step 2 - How to Repair Larger Holes in Lath and Plaster Walls
If the laths are still intact, you can be relieved that no major repairs are needed. To repair a wall in this situation, it is similar to repairing a small hole in the wall as outlined in the previous step. A new coat of render will be needed and this will need to be firmly applied to the laths so that it gets forced between them. When you have applied this set of render and left it for a few days to fully set, you should apply another coat on top of it. This coat should reach a level just below the surrounding plasterwork. When this is dry, apply a finishing layer so that it is up to the level of the surrounding plasterwork. As before, use a low-grade sandpaper to gently smooth the edges of the repairs area so that it blends in with the surrounding wall.
Step 3 - Repairing the Wall if the Laths are Damaged
If the laths are damaged, you may need to replace the entire section. You can use plasterboard for this in many cases though, in the case of some listed buildings, you may be required to repair it using new laths. If the laths are damaged, you will also need to cut back the plasterwork on both sides to the nearest wall frames so that you can replace the laths and effectively rebuild that entire section of the wall. Once the laths are replaced, the repair process is much the same as described in Step 2 albeit working with a much larger area.