- 1). Install drywall panels on the wall surrounding the window as you normally would. Instead of leaving the wood framing next to the window bare, cut and install strips of drywall that match the dimensions of the side framing. These strips take the place of wood window trim and the windowsill.
- 2). Trim the drywall edges where the wall drywall meets the strips inside the window frame. Use a utility knife to shave off the corners where the two panels meet. This is the outside sill edge that you want to be rounded. Shave about 1/4 inch off the outside edge of both panels to remove the sharp corner.
- 3). Measure the length of the windowsill and cut a piece of bullnose corner bead with paper side flaps to match. Do the same thing on the sides and the top of the window, if you’re installing rounded corners all the way around the window.
- 4). Spray corner bead adhesive on the inside edges of the corner bead flaps and position the bead over the corner of the drywall sill you just trimmed. Press and smooth the flaps down.
- 5). Apply premixed joint compound to cover the corner bead with a 4-to 6-inch drywall-taping knife. Smooth the wet compound in a thin coat on the top of the sill and beneath the sill on the wall. Remove excess compound with the taping knife but don’t worry about making it perfectly smooth yet. Let the compound dry completely.
- 6). Sand the rounded corner when the joint compound turns white, indicating that it is dry. A drywall sanding sponge allows you to sand the flat sill and wall areas and offers a little flexibility when sanding over the rounded corner bead.
- 7). Add another thin coat of compound over the rounded sill and on the flat areas adjacent to the sill. Use only enough compound to cover the bullnose corner lightly.
- 8). Let dry, sand and coat again, if desired. The compound contracts as it dries, so it takes multiple thin coats to achieve a smooth surface. After the final sanding, you can paint the sill.
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