Now on to the outside corner beads-
1) – The outside corner joints of a room are actually the easiest to finish, especially if you use a metal bead strip. Cut a strip of corner bead long enough to cover the whole corner top to bottom. Fold the strip in half along the crease and fasten it to the wallboards about every six inches with screws.
Run your 10 inch finish knife down the corner, using it like a straight-edge, in order to ensure the screw heads do not protrude above the wallboard surface. If any screws hit the knife edge, tighten the screw down more or install more screws in the bead.
2) Apply joint compound liberally to the corner, filling in the valley from the corner 4 to 5 inches in from the corner on both walls. Use the metal bead as a guide to level the compound with a 10-inch knife held perpendicular to the wall. Any leftover compound should be used to fill screw dimples. Throw the rest away, then thoroughly wash your tools. Now allow the joint compound to dry 24 hours.
3) – Use a 4.5 inch knife to apply a second coat of compound about 6 inches wide to each side of your outside corner joint. Level it out with the 10-inch knife once again, using the metal corner as to guide you. If you wet the knife down first, then it will smooth better and have less of a tendency to leave behind a wavy effect. Any leftover compound should be used to fill screw dimples. Throw the rest away, then thoroughly wash your tools. Allow the compound to dry completely.
4) – After all the mud it thoroughly dry, run your (dry) 10-inch knife down each side of the corner and knock off any mud that blocks its path as done in Step 3. If the knife hits no more high spots, you are ready for another coat.
5) Repeat step 3, except make this third coat about 8 inches wide on either side of the corner. Allow to completely dry; then knock off any excess mud by running a clean, dry 10-inch knife down each side of the corner.
6) Repeat step 5 except make this fourth coat about 9.5 inches wide on each side of the corner, using slightly thinned joint compound. The corner should now be almost finished.
7) – After the fourth coat is dry, fill in any wavy drag marks by applying a small portion of thinned compound to the imperfection. With the 4.5 inch knife, carefully level it off. Draw the knife perpendicular to the direction of the drag marks, keeping pressure steady on the knife.
8) – Lightly sand the dried compound with 220-grit sandpaper on a sanding block. Vacuum the area of any dust. Stay away from sanding the paper surface of the wallboard as it will leave frayed fibers that look bad even after painting. The corner should now ready to prime with a good latex primer.