If your premixed drywall mud is dried out or has begun to set in the bucket, you may be able to revive it by adding warm water and stirring. However, the first and most important step is to scrape off any hard, dried bits of drywall that may have accumulated toward the top of the container and throw them away because they will not easily reconstitute back into the drywall mud mix.

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. 

Plaster is a pretty tough surface, but it can still sustain damage from things like bumping against it, water issues, dampness, extreme humidity changes, and plain old age. Most plaster walls last hundreds of years and will only be taken down to the base layer when major repairs are required. Being cautious around the wall, not placing furniture against it, and ensuring stable humidity are all great ways to reduce possible damage.

Yes, you can apply new plaster over an existing plaster surface, such as a wall or ceiling, provided the old plaster is in good condition. Before you begin, check the stability of the existing plaster. The surface must be clean, free of loose material, and have no peeling paint. It is also recommended to roughen the surface slightly to promote adhesion. For older plaster, applying a base coat like a PVA emulsion or a specific bonding agent or primer is crucial. This step makes the existing plaster less porous and ensures the new and old layers adhere properly for a smooth, long-lasting finish.