Does a Joint Compound Need to Be Cured Before Priming?

Drying Time

• Before painting, joint compound must be allowed to fully dry. This may take different lengths of time, however, depending on the relative humidity in the air. On a dry day or during the winter, when the air has little humidity, a 1/8-inch thick application of joint compound should be able to dry in one-half hour. If the humidity is high, such as on a rainy day or during the summer months, this drying process can take as much as twice that, or one hour. Extremely thin "skim coats" that are less than 1/16-inch thick, generally take less than 15 minutes to dry, regardless of the relative humidity.

Sanding

• Before painting, joint compound needs to be sanded to obtain an attractive finish. You should sand with a sanding block wrapped in 220-grit sandpaper. The joint compound has to be completely dry before you sand it, however. When you apply the compound, it will have a slightly gray color. As it dries, the gray will fade to dull white. Once the surface is entirely white, you can usually sand the joint compound within five to 10 minutes. You should allow additional time to account for the fact that the surface of the joint compound, when exposed to the air, dries before the lower layers.

Priming

• After sanding the joint compound, you may choose to prime it. This is a good idea, though not technically necessary. When you apply paint, the joint compound soaks up some of it, requiring you to apply additional coats of paint to prevent the appearance of unevenness on the wall. If you apply primer, though, the primer soaks into the joint compound and creates a barrier between the paint and plaster, effectively blocking transmission of the paint into the plaster and allowing you to apply fewer coats of paint.

Painting

• After the primer has cured, you can paint the wall where you applied the joint compound. When using primer, you can use either a sprayer or roller to apply the paint, which speeds up the job and provides a smoother finish. Allowing the joint compound to dry, and then sanding and priming it, produces the results that you would expect from professional house painters.