1. Plasters and renders - what do you need to know?

1.1. External wall renders

Any external wall covering offers a building protection against the weather and prevents moisture getting inside. Along with the roof, it is the most exposed part of any building and must be able to withstand even the harshest and potentially destructive environmental conditions. The choice of facade system, material, and construction is very important and affects the service life of the wall covering as well as the complexity of its maintenance. The most common finish for the external walls in our country is still different types of render. Renders come in a range of different colours and textures, and with various mechanical properties.

1.2. Why opt for render?

Renders are both affordable and enhance the kerb appeal of any property. They are easy to apply so you can even do it yourself. Another advantage is that any surface damage or cracks are easily repaired. Moreover, renders allow artistic freedom in terms of the variety of colours, shades, and designs available, while different application techniques will also result in different textures. In addition, renders come with various added substances that change their chemical, mechanical, and physical properties. In short, the possibilities really are endless.

1.3. Downsides of render

One downside of render is that it can be vulnerable to the weather. Compared to other materials used to cover external walls, render deteriorates faster and must be reapplied at least every 10 years. And forget about rendering your home if your roof doesn’t have eaves as it will be completely destroyed after just a few years. Another drawback is the formation of algae and mould, especially in regions with a lot of rain and a lot of moisture. This can be prevented through the use of algaecides, but these substances are not completely harmless to the environment or human health, and worst of all, the algae will reappear after a couple of years. Algae cause particular problems when there is thermal insulation because they prevent or slow down the passage of heat from the inside out, which results in the render becoming damp and increases the drying time.

2. What types of render are available?

2.1. A wide selection

There are many types of render available: acrylic, silicate, silicone, silicate-silicone, silicone-acrylic, and mineral. In addition to their composition, elasticity, bonding ability and strength, their main point of difference is their water resistance and vapour permeability. Acrylic render is less vapour permeable and waterproof than silicate render, while silicone render is completely water resistant.

Qualitative differences between render types determine which can be used for a particular facade system, because not every render is suitable for every facade system. In fact, it’s the other way around: for each facade system, several types of render are most commonly used. When choosing a render, it is best to follow the manufacturer’s recommendations. In principle, the rule is that there are no special restrictions for walls with EPS insulation (expanded polystyrene), while for those with mineral wool, renders with slightly better vapour permeability are recommended.

2.2. Acrylic render

The emergence of acrylic renders marked a revolution in the render market and the treatment of external walls. Compared to lime and cement renders, acrylic renders are easy to apply, while having excellent properties when it comes to water and vapour resistance. Layers of paint and coatings can be applied to acrylic renders without any limitations and they are relatively stable and resistant to UV rays, while accelerators can be used to improve drying times, especially in winter, when air temperatures are low.

2.3. Silicone renders

Silicone renders are excellent in terms of elasticity, water resistance, and colour durability (with protection against UV rays). In general, they are long-lasting even in the harshest conditions. Large temperature fluctuations that cause ordinary renders to crack will not affect silicone renders as they are breathable and flexible.

2.4 Silicate renders

The important features of silicate renders are the water glass binder and the chemical process of siliconization, which enable a strong bond with the substrate and higher vapour permeability. In addition, due to their high pH, they are fairly resistant to algae and mould. However, they also have one major drawback - they are available in a limited range of colours, which might mean they are not suitable aesthetically speaking.

2.5. Silicone-silicate, silicone-acrylic renders

The advantage of combined renders, such as silicone-silicate and silicone-acrylic renders, is that they have the combined properties and benefits of all the individual renders. Silicone-silicate is ideal in terms of vapour permeability and water resistance, while the others are available in a vast array of colours and have high vapour permeability. Therefore, they offer more possibilities than the single renders.

2.6. Mineral, dry-fix renders

This is a factory-prepared mix of lime, cement, and sand. Their advantage is that they are hydroscopic, which means that they absorb moisture from the air, and thanks to their structure, they transfer water evenly across the entire surface. This allows for much faster drying times and therefore prevents the formation of mould and algae.

2.7. Mosaic (decorative) renders

These renders are manufactured using high quality acrylic emulsion and multicoloured marble grains for the permanent protection of external walls. They are known for their exceptional strength and resistance to damage and stains, even though they are applied in only a thin layer, up to 3 mm thick. The advantage of a composite render like this is the resin content, which prevents the penetration of water, but at the same time allows it to evaporate. Therefore, such renders may be washed and cleaned with a pressure jet, without fear of damage, although, naturally, care must be taken that the jet is not too strong. Due to these properties, mosaic renders are also used as a finishing layer for other facade system substrates – they are easy to maintain and can look great.

3. The rendering process

3.1 When to render?

If you want the rendering to look good and to last, it can’t be done just anytime and anyhow. Cracks and damage most often occur during rainy season, when humidity is highest and when temperatures begin to drop sharply, all of which slows down the drying process. That’s why this time of year is least suitable for rendering the outside of your property and we recommend avoiding it. Generally speaking, ideal conditions for this type of work are when humidity is below 65%. Direct sunlight and wind should also be avoided; this can be done by placing protective netting at a certain distance around the building. In this sense, the period from March to November, when temperatures are between 25°C and 35°C, is the best.

3.2. Preparation of the surface for rendering or plastering

Before rendering an external wall or plastering an internal one, the wall itself needs properly preparing. First, any cracks, knots, or other such issues must be dealt with. The wall is then reinforced with a fibre glass mesh, especially if it is made of different materials. In some cases (such as when the wall is made from autoclaved aerated concrete/foam concrete), it is necessary to apply a primer so the plaster/render adheres better to the existing substrate. A brick wall, however, because it is porous, also needs pre-soaking. After the wall has dried, the actual plastering can begin.

3.3. Plaster mix

The separate components of the plaster can be mixed on the spot in a mixer, although it is more convenient to buy a ready-made mix, though this again still requires making up. It can be mixed manually, if it’s a smaller quantity, usually for internal walls or repairing damage; however, if a large quantity is needed, then a professional machine or cement mixer is used. The rule is to keep mixing until all the lumps have disappeared. In most cases, the manufacturer includes instructions on the correct mix to water ratio to use, as well as how long it should be mixed for.

3.4. Applying plaster/render

Before applying the plaster/render, metal profiles are fixed to the wall; this means the plaster can be applied uniformly across the entire surface. Plaster/render is always applied with a plastering trowel from top to bottom. After the plaster/render has dried, the surface can be treated further depending on the end result you are after, e.g. different textures.

3.5. Layers of plaster/render

One or two coats of render can be applied to external walls depending on the condition of the existing wall surface. Two coats make the end result more durable, while for internal walls, one coat of plaster is sufficient. A scratch coat is first applied to the wall and then the finish coat is applied. Due to the different texture of these two layers, a uniform and beautiful surface is achieved.

4. Making and applying the plaster/render

4.1. By hand

Making and applying plaster/render by hand is a very old, traditional technique. It’s a more complex technique and not all contractors offer it as a service. In the past, plaster composed of sand, water, and lime were mainly used, but their structure has now been improved with the addition of cement additives. The mix is prepared in a mixer. The most obvious advantage of this type of manual approach is the possibility of artistic and original textures and patterns. It can be used for a whole range of finishes: from a rustic, vintage-looking finish, to a rough coating or one with barely noticeable textures. The drawback is the uneven strength, density, and thickness of the plaster/render because it does not always turn out the same.

4.2. Machine-assisted

Machine-assisted plastering/rendering is a much cheaper and faster approach. With the invention of compression pumps, the process of spraying the plaster/render replaced the traditional method. The main advantage is speed - the job can be completed more quickly. However, other plusses include the fact that the plaster/render itself is of better quality and more homogeneous due to the constant mixing, it is always of the same composition, and it is more uniformly distributed. Another major advantage of this approach is that entire surfaces can be covered in one go without any noticeable joins, hence avoiding any peeling which needs to be patched up later. There are several types of plaster/render that can be used with this approach. These differ in terms of their hardness and insulating properties.

5. Types of plaster finishes

5.1 Fine-grained and coarse-grained

This is the most basic categorization of plaster finishes and indicates how it feels to the touch. The fineness or coarseness of the plaster depends on the grain size. For fine plaster, powdery sand is used, while for coarser material, larger fractions are required. The former is usually used for internal walls, while the latter is better for external walls as it is less sensitive to dirt and grime, more resistant, and more durable.

5.2. Structural plaster

This is a type of coarse-grained plaster and is recognizable by its granular structure created by quartz granules. Structural plaster looks lumpy and knotty. It is an excellent choice for those spaces where the walls are not completely flat, because its texture and structure correct the flaws of the wall to some extent. Structural plaster brings a space to life and gives it a tactile richness that ordinary smooth walls don’t have. Therefore, it often only needs whitewashing afterwards since the desired effect will be obtained by the texture of the wall. Its downside is that it is impermeable, making it unsuitable for those spaces where surfaces quickly get dirty or are often damp, i.e. kitchens and bathrooms, because such a wall will be difficult to clean and it will retain grease.

5.3. Textured plaster

Textured plaster has a unique ‘relief’ texture, which means it can be used to imitate broken stone, make a map of the world or a Moroccan wall, or produce the texture of wood. Its plasticity is what makes such finishes possible although to achieve them requires a high level of expertise and artistic and technical skill. It will certainly provide the wow factor in a space where the walls are the most important part of the decor. The limitation of textured plaster is that it requires a completely flat and smooth wall for its application; also, it cannot be used in rooms with a higher than normal level of humidity as it soon absorbs moisture and quickly deteriorates.

5.4. Venetian plaster

This is a multilayer decorative plaster, which is immediately recognizable by the fact that it imitates the texture of marble. It is made of acrylic or natural paints, marble powder and synthetic polymers. It gives an extremely elegant finish and oozes glamour and luxury. Venetian plaster was used back in classical times and is characterized by durability and longevity.

5.5. Mineral plaster

Mineral plaster is a mixture of cement, minerals, and lime. Lime serves as a disinfectant that prevents the formation of algae and mould. Due to its vapour permeability, this type of plaster has moisture-regulating properties thanks to the minerals it contains. Hence, it is an excellent choice for damp spaces. An additional advantage is that it is the most basic and cheapest universal plaster.

5.6. Imitation stone, brick, and wood

Plaster became very popular towards the end of last century because it could be used to imitate more expensive materials such as stone, brick, and wood due to its texture and final appearance. Such designs require a stencil, which is pressed onto the plaster once applied and leaves an impression. The stencil is removed while the plaster is still wet, so that it does not remain stuck to the wall. This type of imitation effect is achieved by embossed plastering, resulting in an uneven surface; hence, it is not suitable for rooms where it might get dirty. Different types of binders can be used for this type of plastering. Silicone plasters are suitable for imitating bricks, organic-based plasters for wood, and marmolite plasters, combined with basic coatings, for flat stone.