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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.

You can usually tell if you have plaster or drywall by touch (plaster is harder, colder, and smoother than drywall) or by tapping on it (drywall sounds hollow, plaster does not). If you still cannot tell, peek under an outlet cover and check the cut edges. The rough edges show the composition of the wall. You will see either drywall (two paper sheets sandwiching gypsum material) or only plaster material (no paper sheets).

Technically speaking, you can use drywall primer over plasters. However, oil-based primers work the most effectively on wall plasters. Meanwhile, water-based primers work just as well with drywalls. Nowadays, there are also universal primers suitable for drywall and plaster. Overall, you will be fine as long as you are using a trustworthy brand.

If your home’s foundation shifts slightly over time, any seams in the drywall represent the weakest point in the wall, leading to cracks. If your home has significant changes in temperature and humidity, this can cause the drywall to swell and contract slightly, causing a crack at the weak point of the seam. If your drywall installer did a poor job securing the panels to the wall studs, this could cause a weak layout and cracks in the seams.