Apply the Corner Bead
Measure a side of the window. Allow a little extra when measuring the corner bead so that you can make 45-degree angles with the paper edges splaying outward. Use a razor knife to cut the paper and tin snips for the metal.
Apply a thin coat of mud to the window edge a little wider than the paper edge of the corner bead. Put the corner bead in place and use the 4-inch blade to scrape away the excess mud so that the paper is flat against the wall and is held in place by the mud. Staple it.
Apply corner bead to the other three edges of the window in the same way. Ideally the beading will frame the window much like a picture frame.
Wait for the mud to dry.
Apply Joint Compound
Use the 4-inch blade to apply a layer of joint compound to the corner beading. Since it is concave along its length it will require a lot of joint compound to fill it. Don't try to do the whole job with one layer or it will take days to dry and will crack in the process. Scrape away as much excess as you can. Wait for it to dry (typically about a day).
Add a second layer of joint compound. The goal now is to begin to feather the edges into a smooth transition with the wall. Use your judgment whether the 4-inch or 6-inch blade is best for this layer. You may be able to finish the job with this layer.
Sand the second layer with 120-grit sandpaper when it is completely dry. By now the edge should be quite smooth with a few pockmarks and imperfections. If there are too many imperfections or the edge transition is too obvious, use the 6-inch blade to add a final layer, filling these imperfections and feathering the edge out beyond the last one you made.
Give the edges a final sanding with 150-grit sandpaper to remove the inevitable ridges left by the blade on the rounded corners.
Things You Will Need
- Rounded corner bead
- Tin snips
- Razor knife
- Tape measure
- Stale gun and staples
- Joint compound
- 4-inch and 6-inch blades
- 120- and 150-grit sandpaper
- Some older corner beads are metal and require screws or nails for installation.