Plaster is an extremely versatile building material made from gypsum mixed with water. Both inside and outside plaster corners may be rounded. Inside corners are rounded typically to ease the act and effectiveness of cleaning, especially in locations where cleanliness is paramount like hospitals and laboratories. Outside corners are rounded primarily for aesthetic reasons. Either type of corner can be created with a few simple tools and some know-how.
Plaster inside corners using your putty knife. Spread the plaster evenly with a smooth one-directional motion. Create a plaster thickness which fades from its deepest at the point of juncture to its thinnest as you move away.
Create a rounded inside corner by running your molder's stick straight down the corner bead while the plaster is still wet. Hold the molder's stick horizontal to the inside corner so it creates the proper groove in the plaster.
Allow the plaster to dry and sand lightly to perfect the smooth, rounded inside corner.
Install a bullnose corner bead or strip lathe to achieve the amount of curve you desire in your rounded outside corner. A variety of bullnose corner pieces are available in any home improvement centre, as are strip lathe and welded wire corner structures which can be custom-made to fit any dimensions.
Use your level or plumb bob to make sure the corner piece is straight before installation.
Install the entire corner in one piece if possible to ensure a straight line and to eliminate the possibility of variations in line along the length of the corner. Use your hammer and nails to attach the corner bead or wire framework in place.
Spread plaster over and up to the corner bead or framework, covering it completely using your putty knife. Wipe away any excess plaster.
Taper the plaster from its thickest point on the corner to a thin layer as you move outward from the junction. Allow the plaster to dry and perfect the finish with sandpaper.
Cleanup of plaster is easy. With a wet rag and a razor blade, most messes can be dealt with.
Plaster outside corners are vulnerable to breakage as they tend to crumble on impact.