Sofia Martinez
Plastic is commonly used in plastering, either as a mould or as an additive to form Modified Gypsum cements, which combine plaster's ease of use with plastic's durability and water resistance. However, plastic can also be detrimental to the plastering process, as cracks can form in the plaster due to plastic shrinkage when an excessive amount of water is lost in the first few hours after application. This is a guide on how to avoid using plastic in plastering and how to prevent plastic shrinkage cracks.
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What You'll Learn
Use Gypsum plaster to avoid plastic shrinkage cracks
Plastic shrinkage cracks occur when an excessive amount of water evaporates from the plaster in the first few hours after application. This results in cracks that often reappear after being smoothed over. To avoid this issue, it is recommended to use Gypsum plaster for internal plastering.
Gypsum plaster is made from the mineral gypsum, also known as Calcium Sulphate. When heated, gypsum loses chemically bound water, hardening into a powder that can be reconstituted with water. This process gives gypsum plaster its unique properties, making it an ideal choice for internal plastering to avoid plastic shrinkage cracks.
Gypsum plaster has a short period of plasticity, during which it can be worked by hand and will maintain its shape, even on vertical or inverted surfaces. This plasticity allows for greater control during application and reduces the likelihood of excessive water evaporation, which causes plastic shrinkage cracks.
To further minimise plastic shrinkage cracks when using gypsum plaster, it is important to protect the plaster from drying too quickly. Avoid direct sunlight and strong winds during application, and initiate plastering at an appropriate time to prevent exposure to unfavourable weather conditions. Additionally, saturate the masonry wall with water before plastering to prevent the brickwork from absorbing water from the plaster.
By following these guidelines and utilising gypsum plaster, you can effectively avoid plastic shrinkage cracks in your plastering projects.
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Avoid plaster drying too quickly
To avoid plaster drying too quickly, it is important to control the suction. Some walls can be very porous and suck the moisture out of the plaster, causing it to dry out. To prevent this, you can use a technique called "painting or flicking". This involves applying water to the wall until it is fully absorbed and starts to run down. In extreme cases, a hosepipe may be required.
Another factor that affects the drying time of plaster is the thickness of the application. While thicker plaster will take longer to dry, it can also lead to other issues such as cracking. It is recommended to apply plaster in layers no thicker than 15mm to 20mm to avoid cracking due to drying shrinkage.
The temperature of the room can also impact the setting time of plaster. If the room is too hot, the plaster will set more quickly. To avoid this, ensure the room is well-ventilated and maintain a comfortable temperature.
Additionally, the water used for mixing plaster should always be clean and cold. Dirty or contaminated water can accelerate the setting time. It is also important to ensure that the plaster is within its best-before date, as outdated plaster may set too rapidly.
To manage the moisture content and slow down the drying process, you can add a small amount of PVA (polyvinyl acetate) to the water when mixing plaster. However, this technique has been debated among plasterers, with some advising against using PVA on plasterboard.
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Avoid thick plaster layers
Applying plaster too thickly can cause a host of issues. The curing process, called hydration, takes much longer when plaster is applied too thickly, as thicker layers retain more moisture. This can cause delays in the drying process and increase the risk of mould and mildew if the area is not properly ventilated.
Thick plaster layers are also more susceptible to cracking and shrinkage once they dry, as the excessive moisture content takes longer to evaporate, causing tension within the plaster. These cracks can impact the aesthetic appeal of your walls and weaken the plaster's structural integrity. They can also be time-consuming to repair and may require additional materials.
Applying plaster too thickly can also make it challenging to achieve an even surface finish. Thicker layers are more difficult to spread evenly and manipulate, resulting in a bumpy or wavy surface texture that may require additional sanding, levelling, or re-application to correct.
To avoid these issues, it is recommended to apply a thin and even coat of plaster, allowing each layer to dry before applying the next. This ensures an even application and minimises the chances of errors. Access to appropriate tools, such as a trowel, is also essential to ensure a consistent, controlled, and professional-looking plaster application.
Additionally, consider hiring a professional with expertise and access to industry-standard tools and techniques. Plastering is a complex job, and a professional can help ensure that the plaster is applied correctly and avoid common issues associated with thick plaster layers.
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Use plaster sand for plastering
Plaster is made from the mineral gypsum, which, when heated, loses water and becomes calcium sulphate hemihydrate, or hydrous calcium sulphate. After it is cooked, it is turned into a powder, and when water is added, it will harden.
Plaster is often mixed with water in a plastic bucket, and it is important to get the consistency right. Plaster has a short period of plasticity, where it can be worked by hand and will stay where it is put. This is when it is at a pasty consistency.
To avoid cracks in plaster, it is recommended to use plaster sand for plastering. Do not use very fine sand that requires more water—this will result in what is known as plastic shrinkage, where cracks appear due to excessive water loss in the first few hours after application. Sand with a fineness modulus of less than 2.2 should be avoided.
Plaster sand is also known as masonry sand or mortar sand. It is a heavy sand that is free of impurities, and it is often used to mix with cement to lay bricks or stones when doing pointing work. It can also be used to mix with cement to create a render or plaster that is used on the interior of a wall.
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Use a plastic cup to control the amount of plaster
When working with plaster, it is important to control the amount of plaster used in the mixture. Using a plastic cup is a great way to do this. Here are some detailed instructions on how to use a plastic cup to control the amount of plaster:
Firstly, prepare your workspace by laying out all the materials you will need, including the plastic cup, plaster, water, and any tools for mixing and measuring. It is also important to have a clean-up area set up before you begin, as plaster cannot go down your sink drain. You can use a three-part cleaning station with two 5-gallon buckets half-full of water and a box with a contractor garbage bag liner.
Next, you will want to measure out your plaster and water. The general rule is to add the plaster to the water, not the other way around, as this can cause clumping and dry pockets. Pour the dry plaster into your plastic cup, which will help you control the amount of plaster you are using. You can then use the cup to pour the plaster slowly into the water, being careful not to dump it all in at once. The speed at which you add the plaster to the water is important to prevent clumping.
Once you have added the plaster to the water, let the mixture sit undisturbed for at least one minute. This process is called "slaking," which allows the plaster particles to become saturated with water. Depending on your preference, you can let the mixture slake for up to 30 minutes. Timing is crucial, so it may be helpful to use a clock or timer.
After the slaking process, you can begin mixing your plaster and water together. Consistency is vital when working with plaster, so it is recommended to write down your technique, including the amounts of plaster and water used, mixing time, water temperature, and the feel of the plaster (smooth or lumpy). This way, you can duplicate your results in the future. Mix the plaster and water until you achieve a consistent mixture with the desired consistency.
By using a plastic cup to measure and control the amount of plaster, you can ensure accuracy and consistency in your plaster mixture. This technique is especially useful when creating plaster molds or casts, as it helps to prevent sticking and ensures a smooth and successful outcome.
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Frequently asked questions
Plastic Shrinkage cracks occur when an excessive amount of water is lost from the plaster in the first few hours after its application.
You can avoid plastic shrinkage cracks by using Gypsum Plaster for Internal Plastering works. Protect the plaster from the sun and wind to avoid drying too quickly, initiate plastering timing to avoid direct sunlight and strong wind on the plastering surface.
Drying shrinkage results from moisture loss after the plaster has hardened. Plaster is bound to shrink and crack. Plaster applied in layers that are too thick will tend to crack.
You can reduce drying shrinkage and the resultant cracks by using plaster sand for plastering purposes and avoiding a rich plaster mix.
USG's Puritan (Number 1) Pottery Plaster is a good multi-purpose plaster. Plaster can also be alloyed with plastic resins, such as melamine, to form "Modified Gypsum" cements, which combine plaster's ease of use with plastic's durability and water resistance.
