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With the template in position, finish by lining the outside with plaster board and then apply a thin skim of plaster over the whole arch. Mark the outline of the arch on the back of the plasterboard using the same tools and techniques you used to mark the outside frames of the template.

Try to make the curve slightly lower than that of the plywood: this allows the plasterboard to overlap the bottom of the template by a few millimetres and makes it easier to plaster the soffit of the arch (fig. 13).

Using a handyman's knife, cut through the cardboard backing and carefully separate the cut out piece from the rest of the sheet. Fix it to the outside of the template using galvanized plasterboard nails and make sure the heads are punched well below the surface, but not right through the cardboard.

Before plastering the arch, check the template carefully and remove rough edges with a piece of abrasive paper. Treat the edges of the plaster­board with a dilute solution of PVA adhesive and cover the joints between the arch and the wall with pieces of decorator's jute scrim.

The top coat should only be thick enough to bring the arch level with the wall on each side. Apply it carefully, first to the outside face of the arch then to the underside, using a metal float. Use a corner shaping tool to finish off the external right angled joint between the soffit and the outside wall (fig. 15). Leave the plaster to dry out for several weeks before redecorating.

How To Repair / Plaster Damaged Arches

To make repairs easy, cut two templates to match the curve of the arch but slightly proud and mount one on each side of the wall