All-purpose joint compound is a building material drywall finishers use to spackle, or mud, a drywall installation. All-purpose joint compound adheres well to most surfaces, so you can use it to make repairs on plaster walls. For example, if your project entails installing a small patch to repair a section of cracked plaster, you can use all-purpose joint compound for all three coats.
To finish a typical drywall patch on a plaster surface, perform the following three coats: Embed paper tape over the edges of the drywall patch, coat the paper tape to hide it and apply a thin polish coat after sanding the joint compound smooth with 100-grit sandpaper. After each coat, you must allow the all-purpose joint compound to dry fully.
Generally, the more joint compound you apply, the longer it takes to dry. Even heavy applications of all-purpose joint compound should dry within 24 hours, but high humidity can increase drying time to 48 hours. If your project is on a tight deadline, set up fans or dehumidifiers to speed the drying process. The compound is dry when it turns bone white.
Using joint compound over plaster walls won't cause any adhesion problems, but the two types of materials might have visibly different textures after paint, making the surface of the wall or ceiling look inconsistent. Plaster often has a rougher texture than joint compound, so you may see noticeable differences across a wall or ceiling if you patch a small area with joint compound but leave the rest of the surface as is.
One way to establish a consistent texture is to skim coat the surfaces around a repair area. Perform your repair as normal. After you finish, add water to the all-purpose joint compound to give it a thin consistency. Apply a light polish coat to the plastered surfaces around the repair area by wiping on the joint compound and then quickly wiping it off. Use a 12-inch drywall knife for maximum coverage. After it dries, sand the area with 150-grit sandpaper.