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This depends on how much plaster is applied and in how many layers, but a decent estimate is around 150 square feet. If you are applying only two coats on a small wall, there is a chance you may only need one or two gallons for the entire project. If you are plastering a large living room wall with high ceilings, using one of the more involved techniques–like Scagliola–that requires several coats, expect the project materials costs to add up quickly. 

Venetian plaster is a highly durable wall finish that becomes a rock-like substance once dried, making it last longer than most other wall finishes. Its lifespan can range from decades to centuries, potentially lasting a lifetime. Key factors influencing its longevity include the quality of the initial application, whether it's properly sealed against moisture, and how the wall is maintained and treated (e.g., how often it is touched or bumped). Due to its mineral content, it can also develop a patina over time.

The short answer is yes, a sagging plaster ceiling is dangerous. Sagging plaster on a wall is something you should fix immediately, but it isn’t a major concern. If the same type of sagging occurs on a ceiling, however, you could be in danger of your ceiling falling in large chunks. When plaster sags, it loses its strength and ability to hold itself up. The bonds that hold it together are strong, though, so if a sagging area falls, it will take large pieces down with it.

While it’s most common to sand plaster dry, it is possible to wet-sand plaster, too. Doing so can reduce the amount of plaster dust released into the air and make it easier to work in smaller, more delicate areas (such as around wall trim). However, some people find that wet sanding leads to more imperfections, which is why larger walls are more often dry sanded to ensure an even finish.