How long does plaster take to dry? A complete guide
Newly plastered walls can make a room start to feel finished, the transformation from the rough blockwork to a flat and smooth surface is spectacular, but that’s not quite the end of the process.
I have recently had several walls plastered. They look great, nice and smooth, and flat but they are still damp and now need to be protected and prepared for decorating, but how long does it take to dry out?
It takes at least 3 days for new plaster to dry when using plasterboard, and backing plaster should be left for 7 days to dry fully. In damp conditions, new Plaster may require longer. Check for dark areas or patches that indicate moisture, Dry plaster will have a light, even buff color.
To find out more, I talked at length to my plasterer who gave me lots of advice about how to long fresh plaster takes to dry out and how to treat new plaster, I also did some research and checked with the manufacturer on best practices, and here’s what I found out.
Types of plaster
There are several different types of plaster that can be used for different situations and conditions the most common ones are:
|
Type of Plaster |
Use |
Setting Time |
Coverage per 25kg bag |
|
Base Filler |
Filling cracks and small holes |
1-2 hours |
N/A |
|
One Coat or Patching plaster |
Patching large areas |
3 -4 hours |
2.25 Sqm @ 13mm thickness |
|
Board finish Plaster |
Top coat for plasterboard & Dry-Coat |
1.5 Hours |
10 Sqm @ 2mm thickness |
|
Multi-finish Plaster |
Top coat for most backing surfaces |
1.5 hours |
10 Sqm @ 2mm thickness |
|
Dry-coat Plaster |
Used for resurfacing areas with a new DPC |
1.5 Hours |
3.25 Sqm @ 11mm thickness |
|
Tough-coat Plaster |
High strength under coat |
1.5 Hours |
3.25 Sqm @ 11mm thickness |
|
Hardwall Plaster |
High impact under coat |
1.5 – 2 Hours |
3.0 Sqm @ 11mm thickness |
|
Browning Plaster |
Under coat for highly absorbent surfaces |
1.5 – 2 Hours |
2.75 Sqm @ 11mm thickness |
|
Bonding Plaster |
Under coat for use on masonry |
1.5 – 2 Hours |
2.75 Sqm @ 11mm thickness |
You can read more about the different types of plaster in our must-read guide to types of plaster, it gives you information about the properties of different types and when they should be used.
How long does Plaster take to dry?
Plaster is mixed with clean water into a thick creamy mixture before it is applied to either a wall or ceiling using a plastering float and other plastering tools.
To achieve the right mixture a 55lb (25kg) bag of plaster needs approximately 2.5 gallons (11.5 litres) of clean water. Once the plaster is applied to the surfaces this water needs to dry out to form a smooth, flat and hard-wearing surface.
For more details on how to achieve the best mix every time read our Guide How To Mix Plaster.
Fresh plaster will take 2-3 days to dry when applied to plasterboard and around 7 days to dry when applied with a backing plaster or undercoat.
If conditions are damp and there are high levels of moisture in the air new plaster can take longer to dry fully. Under extreme conditions of high humidity, fresh plaster can take several weeks before it is completely dry.
Another thing that affects the drying time of fresh plaster, is how thickly the plaster has been applied. A thicker layer will hold more water and take longer to dry out.
Take a look at our beginners guide to plastering for more tips on using and applying plaster in your home.
For example, a new wall that has been built will require a plaster undercoat and then a skim coat, this could be several millimetres thick and will need a lot of time to dry out.
Whereas skimming over an existing plaster surface to cover cracks or repair a patch will be fairly thin, only 2 or 3 mm and this plaster will dry out quickly.
Cooler temperatures will also slow down the drying process and new plaster will take longer to dry, Keeping the ambient temperature above 41 degrees Fahrenheit (5 degrees Celcius) is best for both the strength of the plaster and to reduce drying times.
It is best to avoid plastering in temperatures below this as the mixture will not set properly and the plaster will form a weaker bond to the underlying surface, which will lead to problems later on.
At the other end of the scale applying plaster in hot conditions can also lead to problems. Temperatures above 120 Degrees F ( 49 degrees C) will cause the plaster mixture to lose moisture too rapidly and lead to low strength and the plaster will be impaired.
When drying out, plaster changes colour, from a dark fawn to a light creamy colour. Once dry the surface should be free from dark patches and have an even colour across the surface.
Marbling and natural patterns are common and nothing to worry about, however, small cracks can be an issue and need attention, this can occur if the plaster has dried too quickly or been mixed incorrectly,
For more details about how to Repair cracks in plaster, have a look at our in-depth guide by following the link below:
Where multiple surfaces are plastered at the same time the moisture levels will be significantly increased and additional time will be required to allow the new plaster to dry out.
For example, if you have more than one room plastered at a time, this will mean there is more moisture in the air and the plaster will need longer to dry out fully, allowing for additional drying times is a good idea when planning tasks in case things take longer than expected.
To help the new plaster dry out, keep doors and windows open to allow good ventilation, try to avoid accelerating the process too much, for example, turning the heating up full blast will reduce the drying time but this may be too quick and lead to cracking.
Using dehumidifiers can help to reduce the moisture in a freshly plastered room and speed up the drying process, however, this technique should be used with caution as removing the moisture too quickly can lead to plaster drying out too fast causing cracks and other imperfections, meduim sized dehumidifiers tend to work well as they won’t dry the room out too quickly.
Don’t be tempted to treat or paint new plaster before it has fully dried out as this can lead to issues and even cause the plaster to fall away from the surface below.
Leaving at least a week between finishing the top surface and applying any paint will give the fresh plaster time to dissipate any moisture and allow it to dry.
Keep checking the surface for dark patches and once these have gone leave the plaster to dry for another couple of days to be sure it is completely dry.
How to treat fresh plaster
Newly skimmed walls are a huge step toward completing a room and giving the space a finished look.
Treating fresh plaster properly is of utmost importance in making sure your home looks good and stays that way for a long time.
Before applying any finish to the surface of your walls or ceilings ensure the new plaster has dried out fully.
Fresh dry plaster is extremely absorbent and will suck up any moisture that is applied to it so it is critical that the first coat applied feeds this thirst and seals the plaster.
Applying a ‘mist coat’ of diluted emulsion allows the plaster to overcome this initial thirst and form a good surface that paint or other coatings can bond to properly.
Ignoring the mist coat and using undiluted paint as a base or primer will mean the fresh plaster pulls the moisture from the paint, this means it will dry too quickly leading to peeling paint and poor surface finish ruining the look of your new walls.
Manufacturers like Thistle suggest that plaster can be painted whilst wet so long as a porous paint is used. In my experience, I have never known anyone do this with success and I recommend allowing the plaster to dry out fully before painting.
What is a Mist coat?
Fresh dry Plaster needs thin paint for the initial covering. This is called a mist coat or milk coat and can be created by diluting water-based emulsion with water to thin it down, typically a 50/50 mix or weaker is best.
If you used straight, un-diluted emulsion on fresh new plaster the moisture would be sucked from the paint and it would dry too quickly.
This causes the paint to sit on top of the new plaster rather than to bond to it, which leads to the paint cracking and peeling off, which then needs to be stripped back and start again, which is not fun.
There are products on the market available to buy that do not require diluting, they may be a little quicker to apply, but they work out to be more expensive and don’t tend to work any better than a home mixed mist coat.
Once diluted, the mist paint should have the consistency of milk, allowing the plaster to easily absorb it when applied. How much water needed to achieve this will depend on the paint you use.
You can use any light colour emulsion to form the mist coat, I tend to favour white as it allows any colour to be applied on top, it is readily available and best of all it’s usually cheap.
It’s best to avoid using a PVA solution when treating fresh plaster, this can lead to problems later in much the same way that un-diluted emulsion would, as any future paint is unable to bond to the surface of the plaster and can flake and peel away.
how to apply a Mist coat to plaster
Mist coats can be applied to new plaster by either using a brush, a roller, or by spraying. For most freshly plastered areas using a roller works well.
There are two schools of thought on when to ‘cut in’ the edges of a wall or ceiling, some say to do it first and create a frame around the area which you can then roller up to, others like to leave a border and paint the edges last.
It is down to personal preference, when painting fresh plaster the important thing is to coat it all once with a mist coat.
Fit a medium pile roller sleeve to a roller frame, roller frames are available in many sizes. I use a standard 9” x 1.75” frame as these are fairly cheap and you can find a wide range of roller sleeves.
For large areas, 12” rollers make the job quicker, but using a spray gun may be a better option.
To load the roller with paint follow these steps:
- To use a roller pour out the diluted mist coat into a roller tray so that it fills the paint reservoir.
- Dip the roller with the sleeve fitted gently into the diluted paint,
- Then roll out over the tray ramp to spread the paint evenly around the roller
- Lift the roller carefully up to the area you intend to cover, take care as this will drip.
- Apply gentle pressure and push the roller away until you reach arm’s length
- Bring the roller back over the area you just painted to ensure good coverage
- Then change the angle of the roller and push away again, you are aiming to create a fan of paint in an arc around the area you are standing.
- You can push the roller straight up and down or from side to side but I find that using a random arc gives a better spread of paint.
- When then paint stops covering the fresh plaster, reload the roller by dipping into the paint tray. and start again
- Ensure you cover all of the fresh plaster evenly and use a brush to paint near the edges, it’s important to apply the mist coat to all the fresh plaster.
For large areas applying a mist coat to new plaster can be done using a spray system, either an airless sprayer or with the use of a compressor.
Because the mist coat is a diluted mixture it is perfect use with a spray gun and can be applied to walls quickly and evenly.
Always start with a clean spray pot and ensure the spray gun nozzles are clear and free from old paint.
Ensure that areas like windows, doors, or trims are masked off to protect for overspray, I often find this preparation takes longer to set up than the actual painting.
It is a fine line as to which is quicker a spray gun or roller when painting a large area like walls, ceiling. Using a spray gun means time spent masking up and preparing the area, whereas using a roller is a slower process but does not require as much preparation.
It can come down to just personal preference, and with new cordless, airless spray systems on the market spay guns are becoming cheaper, easier to use, and lighter to handle, making them a more attractive option.
Finishing Coats
Once plastered walls have fully dried and had a good even mist coat applied, they can be finished in any number of ways, the choice of colors, materials, and finishes is endless.
When painting a freshly plastered wall, applying several coats of un-diluted emulsion will create a good solid color, for best effect apply one or two coats then allow to dry and then hold a clean white light near to the surface,
This allows any imperfections to show up by casting a shadow. If the walls are well plastered they should be flat and smooth and any small marks can be locally filled and lightly sanded before applying the final few coats of paint.
Well plastered walls should need very little if any additional preparation such as sanding.
Any minor imperfections can be treated with a light sanding using sandpaper with at least 120 grit, do not overwork the area as this will lead to scratching and remove too much of the surface.
For small particles or dusty areas use a stiff brush to clean them off before applying the mist coat.
It would seem a shame to cover a well-plastered wall, however, wallpaper can be used to great effect. And with a little practice putting up wallpaper is an easy DIY job.
Applying wallpaper can be quicker than having to apply several coats of paint and waiting for them to dry.
Wallpaper can be used to cover plaster that is less than perfect, small cracks, and imperfections will be hidden by the covering.
Take a look at our post on Painting and decorating tips for more great ideas on how to give your home a perfect finish, just follow the link:
Tiles can be applied directly to finish plaster coats, first check the maximum total weight of the tile does not compromise the plaster’s bond to the wall; anything over 20 kg/sqm should be fixed directly to the masonry to avoid plaster and tiles pulling away from the brick or blockwork.
Please note that some of the links in this post are affiliate links, and if you choose to use them we will make a commission, this comes at no extra cost to you.
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.
