How to Tape and Finish Plasterboard Corners
Corners are the most exposed parts of any plasterboard installation. External corners take knocks, internal corners crack if they are not reinforced. Getting the jointing right at this stage saves time and callbacks later. Here's how to tape, fill, and finish both internal and external corners on plasterboard.
Why corners need special treatment
Corners behave differently to flat joints. External corners are vulnerable to impact damage so they need physical reinforcement to hold their edge. Internal corners are subject to movement where two planes of board meet at an angle, so they need a flexible reinforcement that allows minor movement without cracking.
Metal corner tape is a roll of paper joint tape that is reinforced with metal strips to give external corners a solid, straight edge that resists impact. Paper tape, pre-creased down the centre, folds neatly into internal corners and allows the two sides to move independently.
External Corners - Using Metal Corner Tape
Metal corner tape provides a rigid, straight edge that protects the corner and creates a guide for filling. It is applied with filler and finished in the same way as a flat joint.
Start by checking the corner. The plasterboard edges should be clean, undamaged, and tightly butted. If the edges are chipped or uneven, trim them back to a clean line.
Apply a thin bed of joint filler to both sides of the corner, roughly 50mm wide on each face. Press the metal corner tape into the wet filler, with the metal strips facing inwards, centring it over the corner edge. Use your filling knife to press both flanges of the tape firmly into the filler, squeezing out any excess from underneath. Check that the tape is straight and sitting tight to the board on both sides. Adjust while the filler is still wet - once it sets, the corner is locked in.
Apply a fill coat over each side of the corner tape, feathering outward from the corner edge to blend into the flat board surface. Work one side at a time. Build the filler wide enough to create a gentle taper - a joint that is too narrow will show as a hard line at the edge of the tape. Allow to set, then lightly sand and apply your finish coat to complete the joint.
Internal Corners - Using Paper Tape
Paper tape is pre-creased down the centre, which makes it ideal for internal corners. It folds neatly into the angle and allows both sides of the corner to move slightly without cracking the joint.
Apply a thin bed of joint filler into the corner, covering both faces roughly 50mm out from the angle. Fold the paper tape along its centre crease and press it into the wet filler, seating it firmly into the corner.
Using a corner trowel or filling knife, draw along each side of the tape to press it flat against the board and squeeze out excess filler. Work carefully to make sure the tape sits tight into the corner with no air bubbles, wrinkles, or lifting edges. A poorly bedded tape in an internal corner will crack and lift as the building moves.
Apply a fill coat over each side of the tape, one side at a time. Feather outward from the corner to blend into the board. Avoid loading too much filler into the corner itself - the goal is a clean, sharp angle, not a rounded or built-up one. Allow to set, lightly sand, and apply the finish coat.
Finishing Both
Whether internal or external, the finishing process is the same. Once the fill coat has set, lightly sand with fine-grit sandpaper (P180) to remove ridges and high spots.
Apply a thin finish coat of your chosen product, feathering out well beyond the fill coat on each side. Use long, even strokes and work one face of the corner at a time. Allow the first side to firm up before finishing the second - this avoids pulling filler from one side as you work the other.
On external corners, run your finishing knife along the metal edge to keep the corner line sharp and clean. On internal corners, use a corner tool or run your knife along each side separately to maintain a crisp angle.
Allow to dry completely, then lightly sand. The corners are now ready for decoration.
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