How to plaster an arch

Start by carefully removing any existing plaster and external angles around the arch. Use a hammer along with a pry bar or wrecking bar to gently pull away the old materials. If any screws are holding them in place, remove those first. Clear the area back to the framing to make room for your new arch mould and plasterboard.

Start by measuring the width of your wall opening, then cut a piece of MDF sheet into a square using that measurement. Find the centre point of the square and hammer in a screw or nail. To draw your arch, use either a strip of masonite or a piece of string. Tie one end of the string to the centre screw and the other to a pencil, then swing the pencil in an arc from one edge of the MDF to the other. This will give you a smooth, symmetrical arch shape.

Once the arch shape has been marked, cut along the curved line with a jigsaw. Repeat this process to create four identical pieces—two for each side of the arch mould. When all pieces are cut, use sandpaper to smooth the curves and remove any rough spots or bumps. This will help your arch come together neatly in the next steps.

Measure the depth of your wall opening, factoring in the thickness of the MDF sheet and the plaster layers. Cut pine blocks to suit and use them to join two of the MDF arch shapes together, forming one side of the arch mould. Place the pine blocks around the outside edges so you can screw the arch into position. With the mould fully assembled, fit it into the wall opening and secure it in place using timber screws.

Begin by measuring and cutting a strip of plasterboard to line the inner curve of the arch. To bend the plasterboard, score the back every 20 to 30 millimetres using a sharp utility knife. This will allow it to flex without cracking. Starting from one end of the arch, screw the plasterboard into place, spacing each screw approximately 100 millimetres apart. Gently work your way around the curve until the board is fully secured.

Begin by measuring the height and width of your arch. Using these measurements, cut a square piece of plasterboard to fit over the front face of the arch. Position it in place and fix it using screws, spaced approximately every 100 millimetres.

With the plasterboard secured, use a plaster saw to carefully cut out the arch shape from the inside. Work slowly and follow the curve as closely as possible to achieve a clean, accurate cut that matches the arch frame underneath.

To create a clean and durable edge along the outer curve of your arch, install a flexible plastic external moulding. Plastic is ideal for this job because it can be easily notched to follow the curve without kinking. Use a utility knife to carefully score the back of the moulding at regular intervals—this will allow it to bend smoothly around the arch. Once shaped, position the moulding along the edge where the curved plasterboard meets the face of the wall, then staple it directly to the plasterboard, spacing the staples approximately every 100 millimetres. Make sure the arch moulding sits flush and follows the curve evenly to create a sharp, professional finish once plastered.

Before applying plaster, it’s important to prepare the area to ensure it adheres well. Lightly score the painted surfaces near the joints in a criss-cross pattern using your scraper. Then apply jointing tape along the joins and over the moulding edges. This will help create a seamless base for your plaster.

Mix your plaster following the manufacturer’s instructions. If you're using a powdered version, stir it with water in a clean bucket until it reaches a smooth, creamy consistency. Be sure to only mix what you can apply within the plaster’s working time. Using a trowel, apply a thin, even coat across the surface of the arch, starting at the top and working your way down. For the curved edges, switch to a corner trowel to maintain the shape. Take your time and apply the plaster carefully to cover all areas.

Once your first coat is completely dry, scrape back any bumps or high spots with a scraper. Then apply a second thin coat, using the same technique. Let this dry fully before moving on to the final top coat. Feather out the top coat at the edges to blend it into the surrounding wall for a smooth, seamless finish. As always, allow it to dry completely before the next step.

When the final coat is dry, lightly sand the entire area using a sanding block or pole to remove any imperfections. Make sure the surface is smooth and dust-free before you begin painting.