Plaster wall and ceiling repairs in old apartment/condo buildings can be more challenging than plaster repairs in old houses. While houses were built with wood framing and plaster and lathe, high-rises were built with steel, plaster and mesh. Because of this construction method it can be very difficult to replace damaged plaster areas with drywall, as there is nothing in the wall to screw the drywall into.
When the inner layer of plaster is still structurally sound, the best route is to scrape/remove all the damaged top layer of plaster and apply new plaster right on top of the old inner layer.
Using this method, we apply layers of hot mud on the damaged area, until we achieve a uniform smooth finish. Hot mud dries quickly and to a cement like hardness. This makes it the ideal material to bond with the existing plaster and to provide a hard solid surface. While this is the best method to repair old plaster walls, it also presents its challenges. Because hot mud dries quickly, we have a limited amount of time to apply it and smooth it out. Also, because it dries very hard, it is also more difficult to sand, for this reason, we always spend a bit of extra time on the last coat to bring it as smooth as possible.