Corner beads are vinyl or metal strips used to provide a clean line along wall edges and corners. The beads protect drywall edges from being dented and crumbling. For a smooth edge around an archway, use a vinyl corner bead. Its flexibility allows it to be attached to the curvature of the arch. A professional looking installation requires an accurate measurement and a careful application of joint compound.
Measure the length of the arch with a tape measure. Add a few extra inches to this measurement to allow for the bending radius of the bead. Any extra can be cut off after it has been attached to the arch. Transfer the measurement to the corner bead and cut it to size with scissors.
Look at the corner bead and notice that the sides are different. The side with the tabs will be placed against the vertical flat wall. As it is positioned, the tabs spread to accommodate the curvature of the arch.
Drill drywall screws through the holes on the tab side and the side on the inside of the arch to secure the bead. Run a drywall knife along both sides of the bead once it has been attached. If you feel or hear the knife hitting the tops of the screws, this means they should be screwed further into the wall. If the knife snags an edge of a tab, trim the tab with a utility knife. Taking the extra time to do this makes the application of the joint compound easier.
Make a mitred cut at the bottom of the bead on each side of the arch. If the arch sits at the top of two walls, the cut should be made so the bead is flush with the top of the vertical corner beads.
Apply a thick coat of joint compound over the tab side of the bead. The layer of compound should fill the gaps between the tabs and the tab holes. Skim off the excess compound with a clean, wet drywall knife. Blend the outer edges of the compound into the surrounding wall surface.
Cover the bead on the inside of the arch with joint compound using the same technique. Smooth the edge of the arch with a flexible rounding tool. Allow the compound to dry.
Examine the dried compound and gently scrape and knock down any ridges or bumps with a taping knife.
Repeat Steps 5, 6 and 7 to apply a second coat of compound.
Apply a thin coat of compound, if necessary, to fill in any ridges. Immediately skim the compound off with a wet taping knife. Skim in a direction toward the bead instead of along the bead. Allow the compound to dry.
Smooth the area with 200-grit sandpaper and a sanding block. Use a gentle, circular motion. Remove the dust to prepare the arch for painting.