The second coast of joint compound is general called the filler coat. During this coat is where the most compound will be used since you will be filing in the middle and widening the seam. For the second coat you can use either an all-purpose, topping or a setting-type joint compound. I general use the setting-type joint compound if water is available if not I’ll use the all-purpose joint compound. Before you apply the second of mud it is a very good idea to inspect the seam using your mud knife to knock of any ridges or chunks.
Following the same taping sequence as you did on the first coat; fasteners, tapered-edge seams, butted seams, inside corners, then outside corners. To second coat fasteners you only need a thin layer of joint compound to conceal the screw heads, and it’s easiest to mud a row of two or three screw heads in a single strip rather than individually. Using a 6" mud knife, apply just enough pressure to fill the indentation and leave the face of the drywall panel covered with a very thin film of compound. Don’t try to fill the indentation with on thick coat, remember you have to sand, the thicker the mud the harder it is to sand and to achieve a smooth surface.
Tapered-edge seams
Using a 6" mud knife apply joint compound to the center of the seam, the entire length of the seam and into the corners, (this will ensure that enough compound will fill the center of the seam). Now switching to a 12" mud knife place a small amount of joint compound onto the mud knife (the length of the knife), place the knife into the corner and over the center of the seam. Using even pressure with the knife held almost flat against the panel pull the knife along the joint, applying pressure to the outside edges, feathering as you go. The edges should be fairly smooth and feathered when done. Next center the mud knife over the center of the seam using even pressure with the knife held almost flat against the panel pull the knife along the seam. Once done the edges and should be feathered and the center smooth with the joint tape completely concealed. When you have finished the seam, the joint compound should be about 3/16" thick in the center and 10" wide.
Butted Seams
Butted ends of drywall are not tapered therefore when they are tapped a slight bump is created. To conceal the bump it will be necessary to feather the seam much wider than you would normally do so and it may require more than three coats of compound. Before applying the second coat the best thing to do is to lay a straightedge across the seam to see how far you may need to feather the seam. Since the center of the seam will be the high point you only need to apply enough joint compound to conceal the tape and build the joint compound up on the sides. Feather the edges and smooth the center a little at a time. Here is how I usually tape butted seams. Once I have determined how far I’ll need to spread the joint compound I use the appropriate size mud knife usually a 10" knife. Applying joint compound the full length of the knife I then hold one edge of the knife over the center of the joint and with even pressure pull the knife along the seam. Do this for the other side. Once the edges are feathered I center the mud knife over the seam and again with even pressure smoothing the center. You may need this a few times to get the desired concealment.
Inside Corners
I prefer to second coat inside and outside corners after the seams have been second coated as opposed to doing the corners before the seams. This way I do not run the risk of messing up the corners, seams are easier to fix than inside corners. Inside corners are a bit tricky due to the fact that you are working both edges at the same time. Using a 6" mud knife apply a thin coat of joint compound to both sides of the corner. Once the compound has been applied hold the knife almost flat against the wall and turn it out slightly as you pull the knife lightly along the inside edge on one side of the corner. Do one side at a time taking care not to gouge the other side. Now hold the knife a few inches out from the corner and feather the outside edge, do this for both sides. You may need to do this several times before both sides look satisfactory. It will take some practice.
Outside Corners
The easiest of all seams is the outside corner as you may have already noticed the corner bead has a raised outer edge or a lip. This raised edge will act as a depth gauge when applying joint compound. Apply the joint compound with a 10" knife, hold the knife so that it overlaps the raised edge and flat against the wall, with even pressure pull the knife along the corner bead. Feathering the edge and smoothing the center, with the raised edge still visible.