What is Plaster and How is it Used?

In order to create smooth walls within buildings that can then be decorated, plaster will be spread across them to create a consistent look. Normally produced from a combination of cement and sand, this building material has been in use for thousands of years, and today is considered an essential element of most buildings. In this guide from Building Materials Nationwide, we’ll explore the uses of plaster, the many different types available within the UK, and give a quick look at the process for plastering a wall.

What is Plaster?

Plaster is a building material predominantly used to create smooth surfaces on internal walls and ceilings, offering both strength and visual benefits within a property. Plastering has been used in some form for thousands of years, with ancient Greeks using plaster for the exteriors and interiors of temples. In modern buildings, plastering is used to refer to interior work, and rendering is used for the exterior, with a different mix to prioritise water resistance.

Plaster is produced from a mixture of cement, fine aggregates like sand, and water. Plaster and render are both types of mortar, which is used for bricklaying. When buying plaster, you’ll have the option of buying the different ingredients separately or choosing a premade plaster, to which you’ll simply need to add water.

Where Can Plaster be Used?

Plaster is most commonly used on the internal walls and ceilings of a building. It can create a consistent flat layer that can then be decorated. While exposed brick is preferred in some homes, plaster can also be used to hide issues within a wall such as uneven brickwork, or instances where some bricks have been replaced. Plaster can be applied over plasterboards and bricks, and once dried can be painted, wallpapered, or tiled.

There are other uses for plaster worth noting. It can be used for more decorative items within a house, poured into a mould to create detailing for walls and ceilings, such as the patterned arches you might see in a historical church. Plaster is used to produce plaster casts for broken bones, and has also been used for its fire protection, with admixtures available to increase these benefits.

What Types of Plaster Are Used In the UK?

Within the UK, there will be a range of different plaster types available, created with alternative ratios or the addition of admixtures to offer specific benefits to a building. Most of these will be sold as pre-mixed powders, that water will be added to for activation.

Cement Plaster

Traditional plaster is made with the same ingredients as mortar, combining portland cement with sand and water. Plastering sand is available, which is finer than the sands that would be used for brickwork mortar. Cement plaster offers great strength, and can be spread to create a flat surface, perfect for decorating.

• Bonding Plaster: Bonding plaster is intended as an undercoat, offering a great level of adhesion. It can be used over a variety of building materials, bonding well to almost every surface. As it is an undercoat, a top coat of plaster will be needed over it. Undercoats or base coats will need to be scratched to help the top coat bond to them.
• Multi-Finish Plaster: Multi-finish plaster is intended as a topcoat over either plasterboards or undercoats like bonding plaster. It creates a smooth surface on both walls and ceilings, that can then be decorated however you see fit. As it can be applied over a selection of bases it represents a great all-round choice.

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Lime Plaster

Lime plaster is produced from lime, sand, and water, and its use dates back to ancient times. Today it is used within buildings as a topcoat for a few reasons. It offers great longevity, with a reduced risk of cracking. It also offers an attractive natural look, with a soft and textured aesthetic that can simply be left as-is to create beautiful rooms. Colour can also be added through a lime wash, which is a traditional decoration method.

Venetian Plaster

The term Venetian plaster is used for lime plaster that has had pigments added, providing a beautiful coloured appearance within buildings when applied as a topcoat. It will often be finished with a layer of wax. Opting for a specialist plaster like Venetian will bring a heavier cost than more popular methods.

• Finishing Plaster: Finishing plaster is used to create the final surfacing on any wall or ceiling. There are various types available and they will be applied over either a base coat or plasterboard. Coats of finishing plaster will need to be smooth to crate the ideal facing.
• Hardwall Plaster: This is an option for a plaster undercoat, best used on masonry walls, like bricks or concrete blocks. Hardwall plaster is a popular product as it creates an easy base to work from. There can be issues when applying this or any other plaster to walls that are in poor condition. As the plaster will be bonded to the brickwork, crumbling or cracked brick walls can pose issues.
• One Coat Plaster: An alternative to undercoats and finishing plaster is to use a one-coat plaster, which as its name suggests can be applied in a single coat. This is due to its thickness, which allows it to be added in larger amounts than other plasters. It is also a great option for quick repair jobs. 
• Tough Coat Plaster: Tough coat plaster is used as an undercoat, designed to work well in conditions where other plasters would be ineffective. It also offers some protection from fire alongside great impact resistance.

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Browning Plaster

Browning plaster is used as an undercoat and is specifically tailored to be painted, boasting high absorbency. It is also a low-cost option, bringing benefits when used across a construction project.

Plastering

Plastering is a complex process to get right, with many relying on the services of a professional to ensure their walls and ceilings have a level and smooth surface. However, the tools and materials to plaster are available to all, and skilled DIYers can plaster their own houses.

Mixing Plaster

When using a packaged plaster, you’ll be looking for a roughly 1 to 1 mix of powder and water, though you should always check the manufacturer's instructions to ensure this is the case. You’ll need to use a large, clean bucket, fill it with water and add the plaster gradually, mixing as you go. Using a paddle mixer attachment for an electric drill is a great way to save time. You’ll want the finished plaster to be thick enough to stay on a trowel.

How to Plaster a Wall

You’ll need the essential plastering tools, a hawk, a trowel, and a float. All tools alongside the wall will need to be clean of any debris. You should line the floor below with a protective cloth to ensure no plaster gets onto your floor. To help plaster adhere to surfaces, a mixture of PVA glue and water (in a 1:4 mix) should be spread across the surfaces. From there, move plaster onto your hawk board using the trowel. Use the trowel to start spreading plaster on the wall, starting in a lower corner. You should complete the wall in sections, retrieving more plaster from the bucket when needed.

When complete, smooth the whole wall for consistency, scoring it to help the second coat adhere. You’ll then need to add your coat of finishing plaster, using the same method as before. When complete, you can use a float to give a more even finish. Like all cement products, the plaster will take some time to set and should be left undisturbed until it is completely dry before decoration.

Safety Issues While Plastering

All cement-based products come with safety issues to keep in mind during use. Cement dust, whether on its own or included within a pre-mixed plaster, is dangerous to inhale, contributing to several health issues over time. Once wet, plaster can cause serious issues if it makes contact with your skin. You should immediately wash and remove any plaster from your body and should seek medical assistance afterwards. All plaster will be sold with a use-by date that must be adhered to, as it will stop functioning as intended after this date. Out-of-date cement can also release dangerous levels of chromium VI, which can contribute to allergic dermatitis.

Repairing Plaster

Plaster can be repaired, whether it is partially cracked or wholly fallen in specific areas. When refilled with fresh plaster, levelled, and decorated, this can fully restore the look of a wall. Damaged plasterboards can also be repaired using plaster, with a popular method involving holding a solid piece of board behind the hole in the plasterboard and filling the hole fully with plaster, which will dry to give a consistent look.