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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).

Plastering and skimming are two different techniques for finishing walls. Plastering involves applying a thick layer of a plaster mixture—typically containing gypsum, lime, cement, sand, and water—to create a new surface. This process often requires multiple coats and is used for repairing significant damage or creating decorative textures. Skimming, on the other hand, is a technique that involves applying a thin layer of drywall compound or joint compound over an existing surface to smooth it out, covering minor imperfections, small cracks, or uneven textures for a polished look. Plastering is generally more costly and labor-intensive than skimming. Although some people use the terms interchangeably, they refer to different processes and materials.

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.

The best tool for skim coating drywall is a skim coating blade, which can smooth out joint compound in wide stretches. While you could use a smaller drywall knife for skim coating, the smaller size means it’ll take longer to smooth out each section. Instead, consider investing in a skim coating blade for the quickest application.