A rounded or bullnose corner is a revision on the traditional 90-degree corner used on most home's interior walls. It instead uses a rounded cornerbead to produce a gentle transition between two different walls. This presents the challenge of how to plaster a round corner with a flat trowel. The solution involves a specialised trowel called a bullnose corner tool.
Apply plaster to the surface of the flat trowel. Give yourself enough plaster to work without dripping.
Press your middle finger in the centre of the trowel while holding it. Press the trowel, plaster side down, at the top of the wall where it meets the ceiling. The trowel should be just beside the rounded corner on the flat surface.
Pull the flat trowel down the wall maintaining gentle pressure and its location relative to the corner. Work the plaster smoothly along the cornerbead until it is even from top to bottom. You will need to add more plaster to the trowel as you work top to bottom. Repeat this process to apply plaster to the other side of the corner.
Apply some plaster to the bullnose corner tool. Set the corner tool at the top of the wall next to the ceiling and pull down in the same manner used with the flat trowel. Allow plaster to dry for 24 hours.
Sand the plaster until it is smooth and apply another coat, repeating the process from the start.
Plastering drywall seams and corners requires developing a technique that requires some practice before you can perfect it. Extend the plaster 12 to 14 inches from the corner to blend it effectively with the wall.