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Three Coat System

Updated: Jan 29

Application of Natural Lime Plaster

General Guide to Lime Plastering:

Below is a general guide to applying lime plaster. There can be additions and deviations from this due to weather conditions / product used / substrates ect..



Scud Coat / Splash Coat (Optional):

The scud coat, also referred to as the splash coat, is an essential component in lime plastering. This preliminary layer is applied directly to the substrate and serves as a bonding agent for the subsequent coats of plaster. The scud coat is typically composed of a fine aggregate mixed with lime and water to create a slurry-like consistency. It is splash-applied onto the substrate, ensuring complete coverage and establishing a strong bond.


The purpose of the scud coat is to enhance adhesion, promote uniform suction, and create a suitable surface for the following coats of lime plaster. This initial application sets the foundation for a successful and durable plastering project, providing a secure base for the subsequent layers to adhere to.



Drying Time:

Coat

Thickness (mm)

Drying Time (Days)

Scud / Splash

1mm

1 Day

Note: General Guide to drying time is for every millimeter is equal to 1 day of drying time



Scratch Coat:

The scratch coat, which serves as the initial layer applied to the substrate such as bare masonry or laths, plays a crucial role in lime plastering. It is crafted using a coarse aggregate, intentionally creating a rough surface texture that is then vigorously scratched, thus earning its name "scratch coat". This rough and scratched finish serves as a solid foundation, ensuring excellent adhesion for the subsequent coat of plaster.


By creating a reliable key, the scratch coat establishes a strong bond between the substrate and the subsequent layers of lime plaster, contributing to the overall durability and longevity of the plastered surface.



Drying Time:

Coat

Thickness (mm)

Drying Time (Days)

Scratch

10mm

10 Days



Float Coat / Brown Coat:

The float coat, also referred to as the brown coat, is the layer that follows the scratch coat during the lime plastering process.


Like the scratch coat, it is composed of a coarse aggregate. However, the key difference lies in its application and finishing. The float coat is ruled to achieve a relatively flat and even surface, and it is lightly scratched to create a key for the subsequent finish coat. This carefully executed scratching provides a textured surface that enhances the adhesion of the finish coat, ensuring a robust and visually appealing final result. The float coat acts as an intermediary layer, bridging the gap between the scratch coat and the finish coat, contributing to the structural integrity and aesthetic quality of the lime plastered surface.



Drying Time:

Coat

Thickness (mm)

Drying Time (Days)

Float / Brown Coat

10mm

10 Days



Finish Coat / Skim Coat:


The finish coat marks the culmination of the lime plastering process, serving as the ultimate visible surface. Its primary objective is to provide a decorative touch, imparting an aesthetically pleasing appearance to the plastered surface. To achieve this, the finish coat is crafted using a fine aggregate, resulting in a smoother and flatter finish compared to the previous coats. The use of a finer aggregate helps to refine the texture and create a visually appealing surface that is pleasant to both the touch and the eye. With its decorative properties, the finish coat enhances the overall appeal of the lime plastered surface, elevating it to a refined and polished final state.



Drying Time:

Coat

Thickness (mm)

Drying Time (Days)

Finish / Skim Coat

4mm

4 Days

Additional Notes:

As stated at the beginning, this is a general guide and the above is just one example. Below are some top tips for when you are lime plastering.

Apply the plaster gradually in thin coats. Ensure that each coat does not exceed a thickness of 15mm.

Lime plaster, being heavier and slower to dry compared to modern gypsum plasters, poses challenges when applied in thick layers. The excessive weight hampers adhesion, making them more susceptible to shrinkage and cracking, especially in moist conditions.

Wait for the plaster to dry completely before proceeding. Allow sufficient time for each coat to dry before applying the next one.

The recommended drying time is one calendar day per millimeter of thickness. For example, a 10mm coat should be left to dry for 10 days before applying the subsequent coat. Please note that the exact duration may vary depending on site conditions, but it should not be less than 4-5 days in any circumstance.


See more about lime below:


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