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Here is a repair performed on a high end house built in the 1920s. Slow seapage from the chimney and roof had destroyed the plaster ceiling and arched window niche. This was originally two coat work directly over brick or wood lath for the ceiling. Metal corner bead was used on straight external corners. Cornerite (a folded strip of metal lath) was used on straight inside corners. The brick used in this house was a hard, low suction brick. The original (relatively) soft plaster did not adhere very well. Additionally, the joists had sagged, causing much of the ceiling to become loose. Repairs had exacerbated this since new plaster was forced behind the old plaster.
To repair this, water damaged areas were removed down to lath and brick. Later repairs were also removed so that the original plaster could be reattached where intact. The original plaster was reattached using Big Wally's adhesive. Wood lath was renailed since it had become loose. The ceiling was replastered in three coats using gypsum. Although gypsum is not recommended today for use directly over the insides of exterior masonry walls, we used gypsum to match the existing material. The plaster in the window niche was removed to the brick. Three coats were applied, to get better adhesion to the brick. The first coat used USG Wood Fibre plaster. This is a hard gypsum, and sticks well to the hard brick. After this, a brown coat very close to the original base coat was used. The finish was Keanes cement and lime, to match the original hard troweled finish. The curved portion of the arch requires a tool specially contructed for the purpose. The tool is similar to that used for running moldings, except that it pivots about a fixed point by means of a "gig stick." Originally, the entire curved portion of the niche would have been run, not mere the exterior arris as I have done. However, this would have required removal of the entire window casing. For reasons of economy, only the exterior was run. The results are close to rerunning the entire niche. Below are some images.
This is the arch after demolition:
This is the scratch coat:
Here is an image showing the tool in place:
This is the work when finished:
Stucco Classico: Plaster Repair for Pennsylvania