How to Finish Drywall: Third Coat
Third coat also known as the final coat it should be the easiest of the three coats and require the least amount of mud. Before you apply the final coat of mud it would be a good idea to lightly sand all of the seams with 120-grit sanding sponge. The idea here is not sand out all of the imperfections but to sand of and debris, high ridges, bumps and unfeathered edges, thus allowing for a smoother application of the final coat of joint compound.
Following the same taping sequence as you did on the first and second coat; fasteners, tapered-edge seams, butted seams, inside corners, then outside corners. It may not be necessary to third coat fasteners but if so you only need a thin layer of joint compound to conceal the screw heads.
Tapered-edge seams & Butted Seams & Outside Corners
Using a 6" mud knife apply joint compound to the center of the seam, the entire length of the seam and into the corners. Now switching to a 14" mud knife 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 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 without rough or thick spots and the center smooth.
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.