Joint compound is generally used when installing drywall to the exposed studs. This joint compound goes in between all of the joints between each sheet, around corners, and to cover up all of the screw holes. It is applied with a flat-bladed scraper, or drywall trowel and sanded flat with the drywall surface. Joint compound is a mortar-like substance and dries to a light color. Sanding the joint compound does require some special consideration to make sure it is smooth and flush.
Add Enough Compound for Smooth Finish
Before you can sand the joint compound it is important to note that good sanding begins with the compound itself. When applying the compound you want to make sure that there is enough applied for a level surface when it is sanded down. This means that the compound should be several inches on each side of the joint. Smooth it out as much as possible with the scraper to make sanding easier.
Use Light Sandpaper
A coarse grit sandpaper will only scuff up the compound, as well as the surrounding drywall. A 180 to 200 grit sandpaper should be used for smoothing out the joint compound.
Sand Joint Compound by Hand
Despite all of the different electric sanders on the market today, you will get the best look by sanding the compound by hand. There are blocks that are sold that you can wrap the paper around. You can also use a scrap piece of wood if you do not have a plastic holder available. This will allow you have greater control over the sandpaper as you glide it across the drywall. However, a small palm sander can be used for large areas.
Sand Joint Compound in Circular Motion
It is important to sand the joint compound in such a way that it does not damage the drywall. For this reason, you should use a circular motion when sanding the joint compound. Small, tight circles is the best way to both ensure a good sanding surface, but also give you control over the sandpaper's direction.
Sand Out Onto Drywall
The whole reason to use joint compound is for an even, unbroken surface for your walls. The joint compound covers up any seams and screw holes for this type of finish. Sanding the joint compound is one part of the process. You want to smooth the compound until there is no visible height difference or the compound at all. To do this you sand from the center of the joint to the drywall surface and back, keeping the circular motion. This "feathers" the compound into the drywall for a seamless look.
Apply Even Pressure to Sandpaper
The biggest reason to sand the joint compound by hand is that you can apply even pressure along the entire surface of the sandpaper. You do not want to have one part of the compound a little lower than another part. This will create small gullies in the compound that will show up after it has been painted. Even pressure will ensure that these gullies do not show up in the compound.