A Guide to Multi-Finish Plaster

What is multi-finish plaster?

The multi-finish plaster is a favourite for all plasterers. Easy and quick to apply, it is a gypsum plaster which provides a high-quality and smooth finish to ceilings and internal walls. Also, it can be used as a finishing plaster when decorating and is adaptable for surfaces such as flat concrete and low-suction backgrounds like plasterboard. 

Advantages of multi-finish plaster 

1. Versatile: As multi-finish plaster is flexible, it can be used to create smooth finishes on a wide range of surfaces and backgrounds. It is easy to apply by hand and because of its polished texture, sanding is not required. 
2. Reliable setting time: On average, the plaster will take approximately 30 minutes to dry. This makes it a popular choice when working with a tight schedule. It can also help reduce labour costs as it takes less time to dry than other plaster types. 
3. Seals gaps: Because of its consistency, the multi-finish plaster is a great choice for sealing any small gaps and spaces. Once applied, this will limit flanking routes and the transmission of sound. 
4. Improved sound insulation: It can provide additional acoustic properties to the plasterboard and adds further sound insulation. In some cases, the sound insulation improvement has been recorded at 2 decibels (dB). 

Disadvantages of multi-finish plaster

1. Moisture exposure: If multi-finish plaster is exposed to moisture or condensation for a long time, it can cause issues with its strength, durability and loss of adhesion. 
2. Avoid frozen backgrounds: This type of plaster is not suitable for use on frozen backgrounds, but you are able to use it in icy conditions. However, you must ensure the plaster does not freeze once applied. 
3. Too hot conditions: Although extremely unlikely in the UK, Thistle MultiFinish plaster should not be exposed to temperatures above 49°C. Also, if you are applying the plaster in dry and hot conditions, be sure it does not lose the mixing water too rapidly. This would cause the plaster to lose strength and not set properly. 

Tips on how to apply multi-finish plaster

As a rule of thumb, it should always be applied in 2 separate coats and at a thickness of 2mm to achieve the best results. Here are 6 top tips to think about when applying it. 

1. Undercoat preparation

Cement-based undercoats shrink when they are dry and can crack days or even weeks after application. If you apply a finishing plaster before the shrinkage is complete, it is more likely to crack or delaminate, particularly if the undercoat is not keyed enough. Therefore, the key needs to be much better, and the drying time allowance much longer than for gypsum-based undercoats.

2. Apply to a substrate

When applying to a substrate, use multi-finish plaster on the whole surface after the joint treatment has partially set, but not dry. You may find that some joints will experience more movement, such as around window or door openings, on ceilings or under floors that are prone to high deflection, or where board edges lack support.

3. Board backgrounds

A step before applying multi-finish plaster to the plasterboard, be sure to secure flat joints with joint tape or fibre tape. You can fix the tape to the surface of the board before the first layer of plaster. If you decide to use Gyproc Joint Tape, over each joint, embed the tape into the first plaster coat and leave enough plaster to ensure an excellent bond. Once you have applied the tape, press down on it firmly, then cover it straight away with another coat of plaster.

4. How to mix the plaster

When it comes to mixing the plaster, it is important to use clean water as well as clean mixing equipment. This is because any contamination from previous plaster mixes can affect the setting time and strength. Plus, fresh contamination has a greater effect than old. To avoid any issues, wash the equipment just after you have finished mixing.

5. Moisture-resistant grade boards

These boards are manufactured to suit environments with high humidity. Multi-finish plaster (or skim plaster) should not be applied to the material unless the building envelope (such as roofs, windows, doors, foundations and exterior walls) has been made weather-tight. If the plaster has been used on a non-weather-tight building, it can lose adhesion and strength.

6. Wallpapering, tiling and painting

Be sure to check that the multi-finish plaster is completely dry before you start to tile, wallpaper or paint a wall. If you are using wallpaper or paint, you can use most types of each. But it is advisable to check the manufacturer’s recommendations. For tiling, you are able to tile up to 20kg per m2 directly to the multi-finish plaster. However, this will prove difficult if a system includes a bonding agent. Because of the 20kg per m2 limit, you may want to consider tiling straight onto the background instead. 

Below is a table which outlines the drying times for different types of plaster, as well as their uses and coverage.

How much multi-finish plaster should you use?

When you buy a bag of plaster, it may not be enough to finish the project. As a template, one bag is equivalent to 10m2 wall coverage. But to work out how much multi-finish plaster you need, British Gypsum has a Plaster Coverage Calculator which will determine how many 25kg bags to buy. For example, if you are working with a 10m x 4m brick wall, it is recommended to buy 5 bags of Thistle MultiFinish plaster. 

Can I use multi-finish plaster on cement undercoats?

You can use multi-finish plaster, but suction would potentially need to be controlled using a primer to tackle adhesion issues and help retain water, such as the Gyproc Drywall Primer. Cement-based undercoats will shrink during the drying process with some cracking. It can come up days or even several weeks after it has been applied. Be careful not to apply the multi-finish plaster before the shrinkage stage has finished. This is because there is a greater risk of bigger cracks in the finish or delamination. Also, the undercoat needs to be thoroughly keyed to avoid bad cracking and delamination. 

How long does it last once the bag has opened?

Thistle MultiFinish plaster has a shelf life of 4 months. But you may be able to use the plaster past its expiry date by 1-2 months. This is because, during the manufacturing process, the gypsum is heated up to create a compound with fewer water molecules. For context, the bag of plaster is in need of more water and will start to absorb moisture from the environment where it has been kept.

After long periods of time, bagged plaster will start to become stiff because of moisture absorption. The plaster could even clump together from the moisture. Plaster also includes a certain amount of retarder. This is an admixture or additive that slows down the plaster setting time and increases the working time. During the warmer months, it will contain more retarders. Whereas in the cooler months, it will contain less retarder.