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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.

You can technically just paint over hairline cracks in plaster since these cracks are often harmless, but this will mostly be a waste of paint. Paint will only hide the hairline crack for a few months before it shows up again. The best solution is to repair the crack with flexible filler or joint compound before repainting.

You should always hang drywall from the top down. Starting at the top makes it easier to create a straight, tight line between the ceiling and the wall. Any gaps that occur will be at the bottom, where they are easier to manage. You can either cut the bottom piece of drywall to fit the remaining space or cover the gap with baseboard trim. Additionally, starting at the top is physically easier, as you won't have to fit the top sheet into a tight space between the lower sheet and the ceiling, or try to balance the upper sheets on the lower ones while securing them.

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.