Newly plastered walls may give a room a polished look; the shift from rough blockwork to a flat and smooth surface is amazing, but it’s not the end of the process. It can take between 3 and 48 hours for new plaster to completely dry depending on the thickness of the plaster, the ambient temperature, time of the year, central heating system and whether you have insufficient ventilation in your home.
Today, plaster completion time has become a major concern for all types of plaster applications, from home improvement to contract plasterers. However, no two plasters are the same, so you’ll need to know the specifics in order to accurately estimate drying times.
Types of plaster
There are a variety of plasters that can be used for various situations and conditions. The most common ones are:
Base fillers are used to fill bottom cracks and small holes. They are applied in layers, and the backing paper is removed between each layer. It will take an hour or two for the base filler to set.
One Coat or Patching Plaster
Patches, or one coat plasters, are used to cover the surface of an area in need of repair with a cement substrate over the area. These are used to cover larger areas rather than crack fill trowels and regular plaster. This plaster has a setting time of 3-4 hours.
Board Finish Plaster
If you are plastering over drywall and want to give your walls a smooth finish, use Board Finish Plaster. It has all the strength and rigidity of traditional plasterboard, but with added texture for a great finish. plasterboard takes about one hour to set and is ideal for worktops, kitchen units and wall tiles.
MultiFinish or backing plaster produces a hard, smooth regular finish with two or three layers. It has excellent workability throughout. The setting time is approximately 15 – 20 minutes at 20°C . The dry strength of the backing plaster is sufficient for light handling in about 1 hour and for ordinary handling in about 24 hours.
Dry-coat plaster is used for resurfacing areas with a new DPC. It’s specifically designed with a texture that allows you to apply it to the surface in just one coat. It’s best used on Coarse or Medium grade aggregate. Its setting time is about 1.5 hours.
ToughCoat plaster is ideal for backgrounds that have good impact resistance or are strong enough not to require mechanical fixing. Moreover, it provides a very smooth setting surface over masonry substrates such as block or brick.
Hardwall plaster is a masonry undercoat plaster that is most typically used with bricks and medium-density blocks as a background. Hardwall is a wonderful basis to work with, much like the other undercoat plasters we’ve discussed; in fact, its ease of application is one of the reasons it’s so popular.
This is a type of undercoat plaster that is used as a foundation for painting and other ornamental projects. It’s comparable to bonding plaster, but it’s better for surfaces that are more absorbent. They can also be used to ‘build up’ walls and are quite valuable in the construction industry.
Bonding is a type of plaster that is used as a base coat. This refers to the first coat, or undercoat, applied to a freshly painted (or patched) wall. It is scratched with a nail after it has been trowelled off to provide a “key” for the top layer, or finish plaster, to attach to.
Which other factors affect the drying time of a new plaster?
There are several other factors affecting the drying time of a new plaster other than the type of plaster. These includes:
Thickness of clay
The thicker the clay, the longer it will take to dry, since more heat is needed to evaporate the water in the wet clay. Sheet or gauze plaster, which is usually a little thicker than slab plaster, usually takes two days or so to be fully dry. 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.
The porosity of the surface
The porosity of plaster walls has a great effect on plaster drying times depending on the thickness of the new plaster. In general, a 10mm thick plaster will need around 2 hours to dry, while a 30mm thick plaster can take up to 6 hours.
The room temperature
Ambient temperature affects the drying of a freshly plastered room. It is optimal if the plaster is dried in a cool environment at a temperature of 15 degrees Celsius or 60 to 65 degrees Fahrenheit. If the room is too hot, which can be up to 40 degrees Celsius or 105 degrees Fahrenheit, the plaster may crack and lose its flexibility, while if it’s too cold, or below 15 degrees Celsius or 60 to 65 degrees Fahrenheit, the plaster may never dry properly and harden because water cannot evaporate at low temperatures.
The one thing you should do is to open at least one window in each room that has been plastered, this will allow any moisture to escape.
The amount of water in a plaster
The amount of water that a new plaster can hold before drying is due to the type, mix ratio, and the starting and finishing temperature. The amount of water in a plaster affects its drying time because more water means higher water vapour pressure and more evaporation.
How do you know when the plaster is completely dry?
Observe the colour change to determine when the wet plaster is dry. You can tell the plaster is dry when it has a uniformed pale pink complexion.
You must wait until the dark patches or dark fawn has fully vanished to a light creamy colour before you apply emulsion paint onto the plaster; otherwise, paint flaking may occur or mould may grow owing to the salts in the thistle. But what can you do if you want to increase the plaster drying time?
Why is plaster taking so long to dry?
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 takes several weeks before it is dry. When drying out, plaster changes colour, from a dark pink to a light pink colour.
Can you increase the plaster dry time?
There are a few options to speed up the drying process. Some are suggestions, while others may harm your final output. The drying out of the new plaster will be enhanced significantly by increasing ventilation to the area- by opening windows and doors or using heaters. Plant a few heaters around the worked area and allow the plaster dry naturally if you want your wall to dry faster.
Dehumidifiers are used by a few tradespeople, although this is not a good practice. This forces the plaster to dry faster by draining the moisture from the thistle. This can cause the wall to crack and potentially compromise the plaster’s strength.
Heating is a good drying process to remove moisture, although many plasterers would avoid the latter. If you want to eliminate excess moisture once the plaster has dried fully, you can use a dehumidifier. This option should not be used while the plaster is drying.
What happens if you don’t water down paint on new plaster?
It’s especially useful if you are painting new plaster that has not been treated before. Standard emulsion painted onto an untreated wall will not dry well. It is likely to crack and to peel and may not bond properly. If you don’t use a mist coat, you will find that any normal emulsion you apply afterwards sits on the surface.
How to treat fresh plaster
A newly plastered wall goes a long way toward finishing a room and giving it a polished appearance.
It’s critical to treat new plaster properly if you want your home to look excellent and stay that way for a long time.
Make sure the fresh plaster has completely dried before applying any finish to the surface of your newly skimmed walls or ceilings.
Fresh, dry plaster is incredibly absorbent and will absorb any moisture that is applied to it, thus the first coat must satisfy this hunger while also sealing the plaster.
Applying a mist coat’ of diluted emulsion to the dry plaster allows it to overcome its initial hunger and establish a top coat to which paint or other coatings can adhere.
Ignoring mist coats and using undiluted paint as a foundation or primer can result in the paint drying too rapidly, resulting in peeling paint and poor surface quality, wreaking havoc on the appearance of your new wall.
Manufacturers like Thistle claim that you may paint plaster while it’s still wet as long as you use porous paint. However, no one has ever done this successfully, so it is recommended to let the plaster dry completely before painting.
What is a Mist coat?
For the initial covering of fresh dry plaster, the best paint to use is thin paint. This is known as a mist coat or milk coat, and it’s made by diluting a water-based emulsion with water to thin it out, usually a 50/50 ratio or weaker.
On fresh new plaster, if you used direct, undiluted emulsion, the moisture would be drawn out of the paint, causing it to dry too quickly and have uneven brush strokes.
This causes the paint to sit on top of the new plaster rather than attach to it, causing it to crack and peel off, requiring it to be stripped back and started over, which is inconvenient.
There are products on the market that do not require diluting; they may be easier to apply, but they are more expensive and do not perform as well as a homemade mist coat.
The mist paint should have the consistency of milk once diluted, allowing the dry plaster to absorb it easily when applied. The amount of water required may vary depending on the paint you select.
To make the mist coat, you can use any light colour emulsion; however, Most plasterers choose white since it allows any colour to be placed on top, it is readily accessible, and best of all, it is typically inexpensive.
When treating fresh plaster, it’s best to avoid using a PVA solution because it might cause problems later, much as using undiluted emulsion paint, because any subsequent paint will be unable to attach to the plaster’s surface and will flake and peel away.
How to apply a Mist coat to plaster
A diluted mist coat can be applied using a brush, a roller, or a spray gun to new plaster. A roller works effectively for most freshly plastered areas.
When it comes to cutting in the borders of a wall or ceiling, there are two schools of thought: some say do it first and create a frame around the region that you can later roll up to, while others like to leave a border and paint the edges last.
It’s a matter of personal opinion, but when painting fresh plaster, the most important thing is to mist cover it all at once.
Attach a medium pile roller sleeve to a roller frame, which comes in a variety of sizes. Most plasterers use a regular 9″ x 1.75″ frame because they are inexpensive and come with a variety of roller sleeves.
After the plastered walls have dried completely and received a good even mist layer, they can be finished in a variety of ways; the possibilities for colours, materials, and finishes are unlimited.
Applying many applications of undiluted white emulsion paint to a freshly plastered wall will provide a good solid colour; for optimum effect, apply one or two coats, allow to dry, and then hold a clean white light near the surface.
This permits any flaws to be visible by casting a shadow. If the wall is well plastered, it should be flat and smooth, and any small imperfections can be repaired with a light sanding before the final coats of paint are applied.
Walls that have been properly plastered should require very little, if any, extra preparation, such as sanding.
Any small flaws can be sanded out with a mild sanding using at least 120 grit sandpaper; do not overwork the area as this will cause scratching and remove too much of the surface.
The bottom line is that it will take from 3 hours to 2+ days for the plaster to fully dry. That’s a very broad range, so it’s difficult to make a better estimate without more information. There are too many factors at play—temperature, volume, location in the home—to nail down a specific time range with conviction.
The best tip we can offer is to be patient and don’t let your impatience overrule your common sense. Getting impatient and painting plaster before it is completely dry can lead to costly mistakes down the road so be smart and give it time.