Wet sanding plaster after you have made a repair will make the area completely smooth and give you a professional appearance. Wet sanding is particularly important if you want smooth plaster, but maybe don’t have the skill yet to apply the plaster so it dries completely smooth. Wet sanding should ideally take place several hours after the plaster has been installed, so plan on tacking on an hour or so for sanding after you apply the plaster.
The term wet sanding is actually a bit misleading, because there is no sand paper, or other abrasive objects used. Instead, a damp sponge is used to slowly reshape the plaster. You should also keep in mind when applying the plaster that you should still spend as much time as possible smoothing the plaster before you let it dry. Not only will this cut down on the time spent wet sanding, but it will also allow you to practice your plaster application skills.
Step 1: Make sure the plaster is somewhat hard before beginning. It shouldn’t feel wet or sticky, but it should still have a very slight give when you press it. It normally takes a few hours to reach this stage, but the time will vary depending on the temperature and humidity in your house.
Step 2: Dip your sponge in a bucket of room temperature water. Squeeze the sponge so it is only slightly damp.
Step 3: Rub the plaster with the damp sponge with long strokes. Stop when the sponge becomes saturated with plaster.
Step 4: Rinse out the sponge and continue smoothing the plaster with the damp sponge until you have created a complete smooth surface, using long strokes to avoid marks in the plaster.
Step 5: Continue using your sponge to smooth all of the plaster until you have achieved the surface you desire. Wait for the plaster to dry completely, overnight is best.
Step 6: Apply primer and paint to the completely dried plaster walls.