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Although they have similar applications, spackle and joint compound are not the same. Their primary differences lie in their composition, consistency, and ideal use cases.

Spackle is composed of gypsum and binding agents, making it thicker than joint compound. It dries quickly and is easy to sand, which makes it ideal for small wall repair projects like filling nail holes or patching minor damage.

Joint compound, also known as drywall mud, consists of limestone and gypsum. It is primarily used for large-scale projects, such as taping and finishing the seams on newly installed drywall. It tends to shrink more than spackle and can be more difficult to spread.

While both products can be used to fill small depressions and cracks, spackle cannot replace joint compound for taping and mudding new drywall. Conversely, you can often use joint compound for small patching jobs typically done with spackle.

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.

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.

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.