Plastering: Repairing Corners

There are a number of ways of repairing damaged external corners; the method used depends on the degree of damage.

Small chips can merely be refilled with a cellulose filler, while larger, jagged edges may need reinforcing below the plaster coat. This can be done in one of two ways. The first, using aluminium or nylon strip, is best when the surface is to be papered, as the strips fit flush with the plaster. A stronger reinforcement is a round-nosed expanded metal corner piece, which can be plastered over.

To make good small chips, first remove any loose plaster and dust. Mix the filler to a fairly stiff consistency and apply to the corner, proud of the surface area. As the filler starts to set, use a finger to round the corner slightly. Wear rubber gloves. Once the filler is dry, rub down with medium glasspaper to match the surrounding corner angle.

An expanded metal angle bead is fitted in the following way. Hack back about 125mm of plaster, using a bolster and club hammer, on both sides of the corner. Place blobs of plaster at 610mm intervals down both surfaces. Fix the expanded metal corner round the wall angle, checking the true vertical with a straight edge and spirit level, before pressing the edges of the metal into the plaster.

The straight edge is used to adjust the reinforcement, allowing 2mm for the thickness of the finish coat. Render both vertical surfaces to within 2mm for the final finish coat, applying this thinly over the corner nosing, slightly proud of the bead.

Another way involves using battens, but this type of repair will not give extra reinforcement. First, trim back the plaster and nail a batten to the brickwork, so that it protrudes by the thickness of the plaster. With a plumb line or spirit level, ensure thatvit is vertical. Use galvanized plaster nails to fix the batten and place them into the mortar between the bricks; remember to leave the heads proud for easy withdrawal.

Apply a floating coat between the plaster and the batten, rule off, cut back and key as necessary. When the plaster is dry, remove the batten, tapping it lightly to free it from the plaster before withdrawing the nails.

Next, fix it to the adjoining plane and apply a floating coat as before. When this coat is dry, remove the batten and, with a steel trowel, arris the corner. The finishing coat may then be applied.

In the case of hemi-hydrate finish, this may be applied to both sides simultaneously, taking care to apply the slurry evenly. With anhydrous plaster it will be advantageous to use the batten. Fix the batten to overlap one corner of the floating coat, by the amount it was cut back (2mm to 4mm). This time, the batten should be packed out with thin ply, stout card or hardboard, by the amount that it projects.

The finishing coat can then be applied to the corner. When this is polished and set, the batten may be removed, and the other side finished in the same way.

Leave for 24 hours and then round off the corner using fine glasspaper.

10. November 2011 by admin
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