Fixing a Plaster Cornice

Written by samantha volz
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Fixing a Plaster Cornice
A damaged cornice may ruin an otherwise elegant look of a piece. (Jupiterimages/ Images)

Plaster cornices make up the caps and mouldings of many homes, adding decorative touches to areas of the home that may be dull or overlooked. While cornices add elegance and sophistication to decor, the plaster still is susceptible to damage from stress or sudden impact. You can repair these cornices with careful application of adhesive and polystyrene.

Skill level:
Moderately Easy

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Things you need

  • Plastic or metal scraper
  • Tack cloth or vacuum
  • Polystyrene
  • Pencil
  • Utility knife or scissors
  • Plasterboard adhesive
  • Putty knife
  • Rough-grit sandpaper
  • Finishing plaster
  • Sponge
  • Fine-grit sandpaper
  • Paint

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  1. 1

    Use a plastic or metal scraper to remove loose plaster left from the damage. Wipe down the area with a tack cloth, or use a soft-brush vacuum attachment to remove any dust in the area.

  2. 2

    Line up a piece of polystyrene with the damaged area and trace out the shape of the damage.

  3. 3

    Cut the polystyrene with a utility knife or sharp scissors to match the shape of the damage, with a thickness that sets the polystyrene about 1/2 inch from being flush with the top of the rest of the plaster.

  4. 4

    Insert the polystyrene into the damaged area. Mix plasterboard adhesive per product instructions. Use a putty knife to spread the adhesive over the area, scraping adhesive onto the cornice on either side of the damage by about one inch; this holds the polystyrene in place.

  5. 5

    Smooth the plasterboard adhesive as flat as possible with the putty knife. You also may use your finger to achieve a flatter surface; dampen your skin first, and wash off the adhesive with warm water immediately after finishing. When the adhesive is as flat as possible, allow it to dry overnight.

  6. 6

    Sand the dry adhesive with rough-grit sandpaper to flatten it and remove any bumps from the surface. Make sure all areas are completely sealed around the polystyrene.

  7. 7

    Mix a thin batch of finishing plaster and water per product instructions. Spread a thin layer of the plaster over the damaged area with a putty knife to give the repaired area the same texture as the rest of the cornice. Feather the edges of the new plaster a few inches onto the corner itself to make it harder to tell where the cornice ends and repair work begins.

  8. 8

    Allow the plaster to dry for approximately 10 minutes, then wipe the area with a damp sponge to eliminate high points. You need the plaster to be as flat as possible to blend with the rest of the cornice. When the plaster has dried completely, inspect the surface. If necessary, lightly sand it, apply plaster to fill any gaps and allow it to dry again.

  9. 9

    Lightly sand the final coat of plaster with fine-grit sandpaper to remove any small bumps and make the plaster more accepting of paint. Paint the area to match the rest of the cornice.

Tips and warnings

  • For simple cracks in a plaster cornice, use filler putty or epoxy resin to fill in the small damage. Squeeze or carefully push the adhesive into the crack, following product instructions regarding any mixing that needs to be done. When the repair work is dry, paint the area to match.

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