Water is sometimes the culprit that makes plaster crack in the first place, so you need to be sure to fix any leaks or causes of moisture several weeks in advance. Temperature also plays an important role as well. It's best to maintain a room temperature of 55 degrees to 70 degrees while plastering. The room you're working in will need to be at this temperature at least 24 hours before plastering to ensure that the walls are completely dry. It must stay at this temperature during plastering and until the plaster has completely set.
While covering damaged plaster with drywall is cheaper, materials that are period-authentic maintain the historic integrity of an old house. In addition, the plaster will be more durable.
Begin by assessing the cracks and trying to determine possible causes. In this case, there are large cracks that have been repaired previously, as well as mid-size and tiny "spider cracks." Settlement over time is one cause of cracked plaster, but another is key disintegration. This is where the plaster protrudes through the wood lath to create a bond. It can also be caused by poor-quality plaster, water damage or normal wear and tear.
The first step in repairing cracks is cleaning them out. Use a five-way tool or other scraping device (Image 1).
In the case of larger cracks with plaster falling out, remove the damaged plaster to reveal the binding course.
Knock old plaster through keys in the lath to establish a better binding course for the new plaster to adhere to (Image 2).
Large cracks, where the lath and plaster have been pulled away from the wall, can be re-secured using metal plaster washers and ordinary 1-1/2" or 2" drywall screws (Image 1). This secures the existing plaster to the wood lath.
Secure screws and washers diagonally on either side of loose plaster, about 1-1/2" to 2" from the crack (Image 2).
This plaster repair consists of two coats: a base-coat plaster for re-establishing the key-ways for a binding layer, and a top-coat finish layer to provide a smooth surface. (Key ways are spaces between the slats of the wooden lath.) The base-coat is a lime-based plaster, also called "brown coat," that is heavily sand- or silicate-based.
Combine base-coat plaster using room-temperature water, following the manufacturers instructions (hot water can cause the plaster to set up too quickly). Using an electric drill with a paint-paddle, mix the plaster to about a sour-cream consistency and allow mixture to sit for a few minutes as it starts to thicken -- or slake.
Mist the lath with water lightly (Image 1), being careful to avoid over-saturating the wood. This will help to prevent the lath from wicking moisture out of the plaster, causing it to dry too quickly and possibly result in more cracking.
Using a trowel, squeeze base-coat into the cracks (Image 2), making sure to get it into the keys of the lath.
Smooth over the crack and angle off the excess (Image 3).
Depending on humidity, the base coat can take from 2 to 24 hours to set up completely. Allow a couple of days for it to dry completely.
Once the base coat has dried, sand to a smooth finish.
A previous repair to the plaster was made with a drywall patch and gypsum. In this case, use a lime-based plaster because gypsum and lime create a negative reaction. To smooth out the rough patch, we skim-coat the area with more gypsum compound and smoothed down our finish-coat even with the patch, feathering it in for a smooth appearance. The compound was applied using a 12" knife and spread smooth. Once the job is done and the surface has been painted, it the two surfaces should be essentially identical.
The final step is to apply the finish coat. The finish coat is much finer than the base coat, and has a high lime and talk content that gives the plaster a sheen.
Spread the finish coat evenly over dried base coat.
The finish coat can also be used alone to cover small, hair-line cracks.
Polish the plaster repairs by lightly spraying with water and smoothing over with grout float. Once the finish coat has dried, the newly repaired plaster can be given a fresh coat of paint or wallpapered.
Note: This is a summary of steps included in the procedures shown in an episode of Restoration Realities. There may be variations in procedures for your particular restoration project based on the types of materials you select and the nature or extent of your particular project. Always follow proper safety precautions, and read and follow manufacturer's guidelines, diagrams and safety notices that come with materials or products that you select.