How to fill deep cracks in Plaster walls and ceilings
It is a fact of life and physics that buildings move. They move for all sorts of reasons, most of the time it’s due to minor expansion and contraction on warm or cold days or a little local vibration and is generally nothing for homeowners to worry about.
When a building moves it can cause cracks in the smooth plaster surfaces on walls and ceilings, but these unsightly cracks don’t need to be a headache following our simple guide you can repair them in no time and make your home look as good as new.
First, open up the crack and remove loose material with a scraper and vacuum away dust, for deep cracks use expanding foam as a base, wet the area with a sponge, then fill the crack with ‘deep crack filler’, remove excess with a metal scraper, allow to dry and then sand the area with 120 grit paper ready to paint.
This is one of the best ways I have found to repair deep cracks long-term and keep my walls looking smooth for years to come, I have put together a full step-by-step guide in the post below with plenty more great tips including the tools you will need and how to paint the area once filled.
Tools you will need:
Here are my recommended tools for repairing plaster and filling deep cracks and hole in walls and ceilings:
How to prepare the cracked plaster ready for filling
The first thing to do when repairing an area of cracked plaster is to assess how large an area you have to work on, understanding this can make a difference in how you should repair the crack.
Check the plaster running alongside the crack for signs of damage, look to see if it is loose or fully bonded to the walls or ceiling, and any areas that have come away from the substructure will need to be removed as well.
Movement in buildings can cause plaster to ‘pop’ away from the surface below, this is often referred to as having ‘blown’ and will sound more hollow when tapped gently than a solid surface.
You may well be able to move the plaster by applying pressure with your hand or finger or even see a gap between the wall and the plaster near the edge of the crack.
Once you have found the nearest solid surface to the edge of the crack mark an outline with a pencil to show you how big the area is.
I like to use the old DIY mantra:
‘Preparation is Everything’
Protect the local area and flooring around the wall or ceiling you are going to work on. Use dust sheets or thick cardboard to prevent damage to flooring surfaces.
I like to use a 5-in-1 scraper or a hammer and cold chisel to remove any loose plaster and debris from around the crack and between the gap created,
I recommend you wear a face mask, safety glasses, and gloves for this work to protect yourself from flying debris and dust.
Make sure you get back to a good solid surface, repairs are only as good as the surface they are applied to, and just like plaster, repair filler needs a solid, stable surface so as not to fall away from the wall later.
With all the loose and broken plaster removed, give the cracked area a clean with a brush to remove any large bits or chunks of plaster.
I use an old hand brush and a dustpan for this and place the rubbish into a disposable rubble bag, you can find these online or at your local hardware store.
I always run a vacuum cleaner over the area to give it a good clean and remove any dust which will prevent the filler from bonding to the solid surface below.
I have a really useful little Numatic ‘Henry’ Vacuum for DIY jobs just like this, which I keep in the garage and use so as not to mess up the house vacuum cleaner.
How to fill the cracked plaster
For really deep cracks which go right back to the brick behind the plaster or are more than 1″ or 25 mm deep, use some expanding builders foam to take up the majority of the space, this allows some movement, and fills the gap meaning you need less filler and will help prevent the cracks from reappearing.
Cut away any excess foam from the prepared crack using a sharp knife, you should wear cut-resistant gloves to help prevent any accidents. Using the knife make sure to trim the foam back around 10mm below the final surface, this allows a good thickness of filler to be applied, making the surface area strong once dried.
For cracks that are not overly deep less than 1/2″ or 15mm or not very wide, you can skip this step and move on to applying the filler below
Before applying the filling compound, it’s a good idea to wet the area down with water using a damp sponge or spray, making the area damp will help prevent the dry plaster around the crack from sucking away the moisture in the filler too quickly, which can lead to the filler shrinking and not bonding to the surrounding are full.
Using a flexible filing knife or plastic edge, often an old credit card works well, apply some ‘deep gap filler’ to the crack, and always work across the gap so as to work the filler into the area.
If you work the filler along the crack it will have a tendency to pull out of the gap making a mess and the job will take much longer.
Once the gap is filled, the area covered, and before the filler dries out, use a wide metal blade to remove the excess filler and clean up the area I like to use a 6″ jointing knife, This one from Marshalltown is very good and will last you a long time if looked after.
I run the blade at an angle along the crack to help even out the surface, remove as much as possible to reduce the amount of sanding to be done when the filler is dry.
How to long does filler take to dry out before painting?
Water-based fillers like any other type of wall repair, including plaster are activated by water and they need time for the moisture to be released so they can dry out and harden before they can be sanded or painted.
Once I have filled the cracks and gaps, I normally leave the filler to dry out and harden for at least 12 hours, some brands suggest their products can be sanded and finished more quickly so if you are in a hurry it is a good idea to check the brand or manufacturer’s instructions to make sure.
For really deep cracks in walls or ceilings or where you have used a lot of filler on a large area, it can take a few days before it is completely dry and ready to finish.
For more details on drying times for plaster check out our in-depth post ‘How long does plaster take to dry’
Before I sand or paint filler or plaster, I check to make sure it is dry and hard enough so as not to pull out of the crack.
A simple method is to use your index finger or thumb and gently press the surface.
If you are able to leave an impression of your finger or fingerprints then consider leaving the area to dry out for a while longer, I usually leave it to dry for another 12 hours and check again.
How to finish filler ready for painting?
With the cracked area now filled and dry, I like to check it again for any shrinkage or small cracks, often with large or deep cracks in plaster they can need a second run of filler to level the surface,
If this is the case for your wall or ceiling repair, add some more filler to the required areas and leave it to dry just as before.
Once you are happy with the levels, the filler is dry and you are sure that there are no low spots, the next stage is to remove any excess filler.
Hopefully having used the jointing knife to smooth the area there won’t be too much to remove. for walls and ceilings, 120 grit paper works well, it is not so fine that you get an overly polished surface and not too coarse as to create lines.
Gently rub the high spots until they blend into the surrounding area and are as smooth and flat as possible, take your time and use your fingers and eyes to check the surfaces feeling for ridges and edges, and look for high spots.
I use a large hand sanding pad for big areas, as this helps level the whole surface and is pretty quick, I prefer to do this by hand rather than use power tools as it is easier to control the process and get a good surface and helps to keep the dust down.
If you are going to use an electric sander make sure you connect it to a dust extraction system and always wear a face mask to protect yourself.
After I have blended the filled crack into the local area with sandpaper, I use a vacuum cleaner to clear away any dust, I use my trusty Neumatic Henry and a soft upholstery brush attachment.
I keep my Henry in my garage/workshop as a go-to unit rather than use the house vacuum for DIY jobs for two very good reasons:
Firstly – the house vacuum would quickly become clogged and break with all the heavy dust and dirt,
Secondly – and more importantly… my wife would have a field day if I used the shiny new Bissel vacuum cleaner! We have reviews on these and many other tools in our tools review section.
How to Paint filled cracks in plaster?
when painting over cracks in plaster walls or ceilings I treat any filler used the same way I treat new plaster, check out our complete guide on how to treat and paint fresh plaster here
- Allow the filler to dry out fully
- Apply a coat of primer paint that has been thinned out which is often called a ‘mist coat’
- Then paint several coats of emulsion until the crack is covered
I do this because the fresh filler will want to absorb the moisture and if you simply apply thick emulsion it will dry out too quickly and not last very long before it peels away from the surface.
Normally I apply a single, light coat of primer or ‘mist coat’ to the area I have repaired, and then let that soak in overnight before painting the wall or ceiling with emulsion.
I recommend painting the whole wall or ceiling in one go, this may seem excessive but being fussy so and so just painting a patch will leave well just that a patch, the new paint will stand out on top of the old surface, and you will be able to see the new patch.
So do the job properly and paint the whole of area to the next change in surface or edge that way the repair will never be seen and you will have a nice, newly painted wall ready to hang a picture or shelf or simply forget about and enjoy a clean crack frees surface.
This article was written by: Richard Quinton – The DIY Help Desk Owner, Engineer & technical specialist.
Richard is one of the key partners in The DIY Help Desk team. He is a qualified Engineer, writer, and publisher, educated to Master’s level. He is a keen advocate of DIY and home improvements.
Richard enjoys helping others to learn new skills and reach their goals and believes that passing his knowledge and experience on through his writing is an effective way to positively impact the lifestyles and well-being of others on a larger scale.