For those of you who live in houses built many years ago, plaster can be a blessing and a curse. Unlike modern materials, plaster does not off gas, such as with drywall, and is not prone to mold as with some types of drywall. But it can be somewhat difficult to repair, especially if the plaster requiring repair is on brick walls. Just applying a piece of drywall atop the brick and applying joint compound generally does not work. The best way to repair the plaster would be to reapply the plaster as it was originally done. Today there are materials other than traditional plaster to facilitate the repair, namely various types of joint compound.
In the first photo you see a common construction method that was employed when turn of the century (early 1900) houses were constructed. It was common to install wood blocks into the brick to provide a nailable structure for attaching doors and windows. Modern fastening methods did not exist back in the time when nails were the modern fastening method. One problem with using wood such as is seen in the picture is that plaster does not adhere well to the wood and eventually will lose its bond leading to failure of the plaster surface.
Joint compound, or plaster, in and by itself may not bond well directly to the wood blocking (after a century the wood gets extremely dry) thus other methods need to be employed to firmly bond the joint compound, or plaster, to the wood blocking. An excellent method for improving the bond of the compound to the wood is to attach steel (metal) lath (photo 2) to the wood blocking with lath screws. Various types of staples, screws, etc. can also be used to attach the steel lath. Steel lath and lath screws are readily available at most major home improvement stores.
Once the steel lath is attached it is now time to apply the first coat of joint compound or plaster. To improve the adherence of the joint compound or plaster to the wood substrate it would be advisable to wet the wood with water and allow it to absorb into the wood. For the first coat it is best to use a setting type of joint compound (photo 3) to provide a hard durable surface to bond the finish coats of joint compound. When using the setting type joint compound keep it mind that in addition to being hard it is also difficult to sand. As such, it is best to keep it low enough to allow for the application of a sandable joint compound. For those situations where the wood blocking is well below the surrounding brick surface the use of a plaster base coat, available at some home improvement stores, in lieu of the setting type joint compound, is advisable.
The other surface that may need to be repaired, depending on the severity of the damage, is that of the brick. There will be situations where the plaster covering large areas of brick will fail requiring one to replaster the brick. Like the wood blocking the brick will also be extremely dry for the same reason as the wood blocking. To achieve a good bond between the joint compound or plaster the use of a bonding agent (photo 4), found in most home improvement stores, is highly advisable. To apply just follow the manufacturer’s directions. Prior to applying the bonding agent it would be advisable to wet the surface of the brick to add moisture to brick for better adhesion of the bonding agent, as recommended previously for the wood blocking.
When the bonding agent has set (normally this is when it is tacky to the touch) it is time to apply the setting type joint compound or plaster. As with the wood blocking, apply the compound (last photo) so not to leave high spots that would require sanding. After you have applied the compound to the entire surface you can now apply one or more coats of a sandable joint compound. Once you have completed applying the compound sand the surface till in is flat and smooth and apply a quality primer to protect your hard work.