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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.

If your home was built before the 1940s and was not remodeled in the past 50 or so years, you probably have plaster walls. Aside from assuming, there are some practical ways to tell if you have plaster walls, including the following: If you knock on the wall and it feels solid, not hollow, if you can’t insert a thumbtack easily, if there are cracks that look like spiderwebs, or if your home has curved, arched, or swirled walls, doorways, or window openings.

If you have an unfinished basement or attic, you could examine the backs of any finished walls to check, too.

Plaster is denser, harder, and more porous than drywall, so it absorbs moisture differently. This means you’ll often need more prep work, such as thorough sanding, patching, and priming with a high-quality bonding primer. Additionally, you may need more coats of paint to achieve even coverage. While the process takes more time, it helps ensure a durable and professional-looking result.

While you can put new drywall over old drywall, it’s not always the best option. In some cases, drywalling over drywall can make it harder to find wires and pipes behind your walls and conceal more serious problems with your walls. It can also make the room smaller. On the plus side, putting drywall over drywall can improve soundproofing and insulation. Plus, it’s often less expensive than tearing down and adding new drywall.