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Plaster ceilings can sag for several reasons. In some cases, the plaster and other building materials shift and come apart over time, which is why you’ll commonly find drooping plaster ceilings in older homes. Improper installation, foot traffic, and water damage—often from a leaky roof or a broken pipe—can also cause plaster ceilings to sag.

There are six levels of drywall finish, from 0 to 5, that refer to the amount of work performed during the finishing process.

Level 0: Drywall is hung with no finishing work.

Level 1: Tape is placed over the seams between drywall sheets.

Level 2: A thin layer of mud is applied over the tape and screw holes. This level is common for utility rooms or walls that will be covered.

Level 3: A full coat of mud covers the screw holes and tape, but the surface is not sanded. This is often used in garages.

Level 4: This is the most common finish for visible interior walls. It involves applying two coats of mud over seams and screw holes, with sanding between each coat, creating a smooth surface ready for flat paint.

Level 5: This is a high-end finish for luxury homes. It includes full taping, mudding, and sanding, plus a final skim coat over the entire surface before painting.

You should wait at least 24 hours for drywall mud to dry before sanding or applying another coat. However, the exact drying time depends on several factors, including the thickness of the application, room temperature, humidity, and the type of compound used. Thicker layers, or projects in high humidity, may require 48 hours or more. Lightweight compounds tend to dry faster than heavier, all-purpose ones. You'll know the mud is ready when it feels completely dry and firm to the touch and has changed to a consistent light color. Sanding too early can cause smudging or gouges, so patience is essential for a smooth finish.

You can put certain types of plaster over other types, but doing so isn't always the best idea. For example, putting a coat of gypsum over lime plaster is possible, but doing so will limit the lime plaster's breathability and potentially affect its performance.

Since cement plaster tends to leave a textured surface, no matter how much you smooth it, some people prefer to use gypsum plaster as a topcoat over cement plaster, creating a smoother surface.