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Should you discover cracks or patches of loose plaster on the internal walls of your home (usually found when you redecorate), you will have to repair the walls before painting or wallpapering.
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Small cracks can be repaired using an appropriate flexible filler, then sanded to a smooth finish. Larger patches of damaged or loose plaster will require re-plastering to obtain a suitable surface for decorating or painting over.
Achieving a good finish when plastering a large area, such as a complete wall or ceiling relies on the plasterer having a good technique and a great deal of practice. Fortunately, patch plastering can be carried out by a competent DIYer.
There are various plasters available for the job (see different types of plaster). You could use Browning or Bonding plaster with a topcoat of multi-finish for the repair, or as I prefer to use, one coat plaster which is available from the larger DIY stores in various sized bags from around 2.5 kg to 12.5 kgs, so you don’t have to purchase a 25 kg bag of plaster, of which 3/4 will end up at the local tip as its ‘shelf life’ is not very long.
One coat plaster also has the advantage of drying to a white finish.
Safety tip: Mix the plaster, preferably outdoors wearing a face mask to avoid breathing in the dust.
To carry out the repair, start by placing a dust sheet or old towel to protect the floor. Remove any bits of loose plaster and brush away any dust using a soft hand brush or old paintbrush.
The wall you are going to plaster over will be dry so splash or flick a little water on it to wet it down (you don’t have to drown it!). This is done to stop the brickwork from absorbing the water from your new plaster too quickly.
Wait a couple of minutes for the water to soak into the wall.
The plaster can now be applied to the wall. If the patch has a depth of around 12mm (1/2″), you should be able to plaster it in one go, if it is deeper, you may have to make two applications, otherwise, the weight of the wet plaster will drag itself away from the wall.
If you have to apply the plaster in two applications, when the first has dried out and has become firm to the touch, scratch the surface with (for example) a large kitchen fork (a plasterer would use a scarifier to do this). This helps the second application of plaster ‘key’ to the first.
Apply the plaster using your plastering trowel until it is level with the surrounding plaster. Remove any excess plaster around the patch with the edge of the trowel. Allow the plaster to dry out a little (until it is reasonably firm to the touch), flick a little water onto the new plaster and clean your trowel with water leaving it wet.
Smooth over the new plaster with the trowel whilst continuing to flick water on it. Doing this will produce a much smoother plaster finish, this is sometimes referred to by plasterers as ‘bringing the cream to the top’, a phrase which will become self-explanatory when you do this.
Let the plaster dry out thoroughly and any excess plaster can be smoothed flat using a sanding block (a short piece of 3″ x 2″ 75mm x 50mm timber with sandpaper around it will do).
Before you paint over the new plaster apply an undercoat, or if you are wallpapering, apply one or two coats of wallpaper adhesive to prime and seal it.
Tools required for repairing plaster
- Plasterers trowel
- Two buckets
- Face mask
- Hand brush
- Old paintbrush
- Mixing stick
- Dust sheet or old towel
- Kitchen fork or scarifier
- Sanding block (if required)
Check out our list of essential DIY tools.
Video: How to Patch Plaster a Wall
Video Credit – B & Q