Painting some surfaces doesn't require priming or other extensive prep work. Fresh plaster surfaces, however, almost always need proper conditioning before painting. Prepping and priming new plaster is time-consuming, but a professional-looking finish is well worth the extra effort.
Consult the plaster container to see how long it takes to dry. Plaster is dry to the touch several hours before it is fully cured and ready to paint.
Go over the plaster with 150-grit sandpaper. This medium-grit sandpaper helps remove rough patches from the new plaster surface.
Sand the plaster again with 300-grit sandpaper. This extra-fine sandpaper takes off a very thin layer of plaster to create a smooth surface.
Wipe down the surface with a damp rag. Don't saturate the rag, as excessive moisture will damage new plaster. Get it barely wet enough to remove the fine sandpaper dust.
Brush general-purpose interior primer over the plaster patch. Use a paintbrush for small surfaces or a roller for larger areas. In most cases, primer isn't needed for the whole wall if it's already covered in latex or oil-based paint.
Paint the primed plaster after it is fully dried. Don't use the same brush or roller that you used with your primer. A fresh brush will be free of unwanted residue, helping you achieve a more accurate colour match. Primed plaster patchwork and existing painted walls may need just one coat, but you can apply a second coat if necessary after the first one dries.