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Plaster that sags or bulges, shows extensive deep cracks, or is crumbling from water damage or simply time may need replacement. Plaster walls can look a little “worn” for a long time before they need replacing. The only real way to know if it needs a replacement is to hire a professional to assess the condition of the plaster.

Yes, you can install drywall over plaster, but there are important factors to consider. While it can create a smooth, flat surface, it is often not the best approach. Installing drywall over plaster will increase the wall's thickness, which can cause issues with existing window and door frames that may no longer fit. You might also need to adjust power outlets to keep them accessible. A better, though more expensive, option is to remove the old plaster and lath entirely before installing new drywall. Drywalling directly over plaster seals in the old material, and if any problems arise later, you will need to remove both layers down to the structural beams for repairs. Given the potential complications, this job is best handled by professionals.

You can put drywall mud over paint to even out the surface or entirely resurface it. Sand the surface with 280-grit sandpaper, vacuum the debris and dust with a shop vac, and clean it with a slightly damp cloth. Allow the surface to dry. Apply drywall primer, especially if you’re skim-coating an entire wall. Apply the right type of drywall mud over the surface and smooth it with a trowel. Allow the layer to cure fully, then sand it before applying another layer or painting it.

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).