Surface Preparation for Different Types of Plaster: Gypsum, Lime, and Cement

Surface preparation is an essential yet often overlooked stage in any plastering project. Whether you’re working with gypsum, lime, or cement plaster, how well you prepare the surface can determine the final quality and durability of your work. Each type of plaster has unique requirements when it comes to surface preparation. By using the right tools and techniques, you can ensure proper adhesion, even drying, and a flawless finish.
In this guide, we’ll walk you through the key steps of surface preparation for each type of plaster, incorporating REFINA’s premium tools to make the process more efficient and professional.
Why Surface Preparation is Critical
Surface preparation is about creating the perfect canvas for plaster application. Without thorough preparation, even the highest quality plaster can fail, leading to issues like poor adhesion, cracking, and uneven surfaces. The ultimate goal of surface preparation is to ensure that the plaster bonds well to the substrate and that the surface remains smooth and stable throughout its life. Different types of plaster—gypsum, lime, and cement—require specific preparation steps to accommodate their unique properties. Using specialised tools from REFINA can simplify the process, making it easier to achieve professional-level results.
How to Prepare Gypsum Plaster Surfaces
Gypsum plaster is a quick-setting material made from gypsum, a naturally occurring mineral composed of calcium sulfate dihydrate. When mixed with water, sand, and other additives, it forms a paste that can be easily applied to walls and ceilings, enhancing surface quality. In the UK, multi-finish plaster is the most popular type of gypsum plaster, valued for delivering high-quality finishes across a wide range of plastering applications.
Once fully cured, gypsum plaster creates a tough, durable surface that resists cracking and withstands light wear. As a gypsum-based product, multi-finish plaster also boasts excellent moisture-regulating properties, allowing walls to “breathe” and reducing condensation, which helps maintain a healthy indoor environment. To ensure proper adhesion and even drying, it’s important to thoroughly clean and prime the surface and control moisture levels during application.
Step 1: Surface Cleaning
The first step in preparing a wall for gypsum plaster is to ensure the surface is clean and free of any dust, grease, or loose particles. Even small contaminants can prevent the plaster from bonding effectively, leading to surface imperfections.
For this task, a render scraper or wire brush is ideal. REFINA’s sharp-toothed scrapers are specifically designed for efficient surface removal, while wire brushes are excellent for eliminating old mortar, plaster, and paint.
The precise scraping action creates an optimal texture for the base coat, improving the adhesion of topcoats or finishes. This ensures a long-lasting, professional-grade result.
Step 2: Priming with PVA and Bonding Agents
Priming is a crucial step, particularly when applying gypsum plaster to porous surfaces like brick or blockwork. Applying a PVA solution seals the substrate, preventing it from absorbing moisture too quickly from the plaster. This ensures that the plaster sets evenly and remains bonded to the surface. Use a REFINA Premium Water Brush to apply the PVA solution evenly across the wall. This high-quality brush allows for smooth, streak-free application, making sure the primer reaches every part of the surface, especially on rough or uneven textures.
We recommend using SBR (Styrene Butadiene Rubber), an additive that boosts the performance and durability of plaster. It enhances adhesion, ensuring the plaster bonds effectively to difficult surfaces such as smooth concrete or old masonry. SBR also improves water resistance, minimises the risk of cracking, and adds flexibility, helping the plaster withstand movement or shrinkage.
SBR can be applied as a primer or mixed with cement as a bonding agent, reinforcing the bond between plaster layers for a longer-lasting and more durable finish. Furthermore, on high-suction surfaces, particularly dry substrates, using SBR (Styrene-Butadiene Rubber) as a bonding agent is highly effective in reducing suction.
A bonding agent may be necessary in cases where the surface is particularly smooth or non-porous, such as concrete. This enhances the adhesion between the plaster and the substrate, preventing peeling or cracking after drying.
8mm sharp & twisted
10″ render scrapers
Blue grip & stainless steel prongs
4″ – 7″ water brushes
3 bristle options available
Excellent water retention
Step 3: Dampening the Surface
Gypsum plaster should not be applied to a dry surface as it will absorb water from the plaster too quickly, causing cracking. Lightly spray water on the wall using a misting spray bottle or a sponge. The surface should be damp but not saturated with water.
However, please note that this step isn’t required if you’re applying Gypsum plaster to plasterboard.
Step 4: Application of Gypsum Plaster
After the surface is prepped and primed, it’s time to apply the gypsum plaster. For a professional finish, we recommend using REFINA FINATEX-1 laying on trowels for the first coat. These trowels are designed with a 0.65mm stainless steel blade and machine-chamfered side edges, ensuring exceptional precision and quality throughout the application process.
After applying the first coat, it’s important to level the plaster. There are various trowels to use for this process:
- SUPERFLEX Trowels – With a flexible blade, these trowels provide greater coverage using less pressure, thanks to their lightweight stainless steel construction. Perfect for achieving smooth, even results with minimal effort.
- SUPERSKIM Spatulas – These incredibly lightweight spatulas are equipped with a 0.3mm stainless steel blade and an ergonomic ‘Z’ profile frame. The True Edge design ensures even pressure distribution across the blade, delivering flawless flattening and finishing every time.
- X-SKIM Spatulas – Featuring an interchangeable blade and True Edge technology, X-SKIM spatulas ensure consistent pressure across the entire surface. Among the lightest spatulas available, they are highly efficient and easy to handle.
- Flexible Skimming Spatulas – These spatulas are lighter than traditional trowels, making them ideal for large surfaces and ceilings. They reduce user fatigue while providing excellent control and coverage.
When it comes to flattening, finishing, and polishing stages, the SUPERFLEX finishing trowels are essential. These trowels have flexible stainless steel blades, which provide excellent control and precision when applying and smoothing the plaster. The blade’s flexibility allows for easier manipulation, ensuring that the plaster is evenly distributed and flattened, resulting in a smooth, flawless surface.
We recommend applying a minimum thickness of 6mm for gypsum plaster to prevent cracking and debonding.
The original flexible plastering trowel
Radisued corners
Finished blade edges
Sizes: 7″ – 44″
Interchangeable blades
TRUE EDGE™ for even pressues along the entire blade
Sizes: 7″ – 44″
Interchangeable blades
TRUE EDGE™ for even pressues along the entire blade
Key Features of Gypsum Plaster:
- Quick Setting: Gypsum plaster sets quickly, allowing for faster application and finishing than traditional lime—or cement-based plasters.
- Smooth Finish: It can be applied in thin layers, resulting in a smooth and aesthetically pleasing surface, making it ideal for decorative finishes.
- Fire Resistance: Gypsum is naturally fire-resistant, providing an additional safety feature in building applications.
- Sound Insulation: Gypsum plaster offers good sound insulation properties, helping to reduce noise transmission between rooms, especially when paired with acoustic plasterboard.
- Ease of Application: It can be easily mixed, applied, and worked with, making it a popular choice among plasterers and builders.
- Versatile: Gypsum can be used for a variety of applications, including interior walls, ceilings, and decorative mouldings.
- Low Shrinkage: It exhibits minimal shrinkage during curing, reducing the risk of cracks and ensuring a stable surface.
Lime Plaster Preparation
Lime plaster is prized for its breathability and ability to accommodate buildings’ natural movement, making it ideal for older structures and eco-friendly projects. However, lime plaster requires careful surface preparation due to its slower drying time and reliance on carbonation for curing. Ensuring the surface is stable, properly textured, and correctly moistened is essential for successful lime plastering.
Step 1: Cleaning & Dampening The Surface
Just like preparing a gypsum surface, use a wire brush or scraper to remove any loose or flaking material. For surfaces with heavy dirt or grease, wash them with water and a mild detergent, then rinse thoroughly.
Lime plaster requires a moist substrate to carbonate properly, but too much moisture can cause sagging and slow the curing process. To maintain the perfect level of moisture, lightly dampen the surface to prevent excessive moisture absorption. Keeping the wall evenly moist ensures that the lime plaster dries consistently, preventing cracking and improving durability.
Step 2: Roughen Smooth Surfaces
If the substrate is too smooth, like concrete, it may need to be roughened to improve the mechanical bond for the lime plaster. This can be done by lightly scoring the surface with a stainless steel scarifier, which is perfect for creating texture on base coats and backing plasters.
Step 3: Pre-Treating with Limewash
Before applying lime plaster, it’s important to prepare the surface with a limewash or limewater solution. This helps the plaster bond to the wall and begins the carbonation process, which is critical for lime plaster’s long-term durability. REFINA’s brushes are perfect for applying lime wash evenly across the surface. The soft, durable bristles make it easy to cover large areas without oversaturating the wall, ensuring that the surface remains primed for plaster application.
Step 4: Applying Thin Coats with Plastic Trowels
Lime plaster is applied in thin coats, each requiring time to carbonate before the next layer is added. We recommend using the FINATEX-1 for applying the plaster, and REFINA’s plastic trowels for the finishing stages. These are specifically designed for delicate materials like lime plaster. Their lightweight, flexible design reduces drag during the application, allowing you to spread the plaster smoothly and easily over the surface. These trowels are also ideal for use on curved or uneven walls, ensuring a uniform finish even in challenging areas.
The first coat of lime plaster is usually referred to as the scratch coat and should be around 10-15mm thick. Before the scratch coat dries, create horizontal and vertical scratches on the surface to provide a key for the next coat. This process should be repeated with each layer of lime.
Let the scratch coat cure for a few days, keeping it moist by misting it with water if necessary. This helps prevent cracking and ensures proper hydration of the lime.
Step 5: Smoothing & Keying
To achieve a flat, even finish on larger surfaces, lime plaster should be smoothed using straight edges or levels. These tools help eliminate high spots and fill in low areas, ensuring that the plaster is applied consistently across the entire surface. The use of straight edges is especially important when working on large or uneven walls, as they help create a professional, uniform finish that will carbonate properly as it cures.
Key Features of Lime Plaster:
- Breathability: Allows moisture vapour to pass through, preventing condensation and dampness while regulating indoor humidity levels.
- Flexibility: Accommodates minor building movements without cracking, making it ideal for older or more dynamic structures.
- Durability: Long-lasting and resistant to weathering, lime plaster can endure for centuries, especially in properly maintained environments.
- Eco-Friendly: Produces a lower carbon footprint compared to cement-based plasters and is fully recyclable, making it a sustainable choice.
- Natural Antibacterial Properties: Lime’s high pH levels naturally inhibit mould and fungal growth, contributing to a healthier living environment.
- Thermal Performance: Offers good thermal mass, helping to regulate indoor temperatures by absorbing and releasing heat gradually.
- Self-Healing: Lime plaster can “self-heal” small cracks over time through carbonation, reducing the need for frequent repairs.
- Fire Resistance: Naturally fire-resistant due to its mineral composition, offering an added layer of protection to buildings.
- Compatibility with Historic Buildings: It is ideal for restoring and preserving historic structures, as it matches the materials used in traditional construction techniques.
Preparing for Cement Application
Cement is known for its durability and strength, making it a popular choice for external applications or areas exposed to moisture. However, because cement is denser and more rigid than other types, it requires careful preparation to prevent cracking and ensure long-lasting results. Surface cleaning, the application of bonding agents, and proper moisture control are all key steps in preparing for cement plaster.
Step 1: Cleaning The Surface
Similar to the first steps for both gypsum and lime-based plasters, applying cement plaster follows a similar process. The surface must be thoroughly cleaned and textured to ensure proper bonding of the cement plaster.
Step 2: Applying Bonding Agents
If the surface is too smooth or non-porous, a cement-based bonding agent can improve the plaster’s grip. Bonding agents can be applied using a brush, roller, or spray machine if you need to cover large areas quickly.
Designed to deliver exceptional performance and control for professional plastering applications.
Step 3: Dampening The Surface
Cement plaster dries rapidly, and without sufficient moisture, it can shrink and crack during the curing process. Lightly mist or dampen the surface before plastering but ensure it’s not overly wet, as excess water can cause the plaster to slide off or crack after drying.
Step 4: The First Coat (Scratch Coat)
The first coat, known as the scratch coat, is essential for sand and cement renders. The correct mixing ratio for this layer is 4 parts sand to 1 part cement. For applying cement plaster, REFINA’s FINATEX-1 trowels are an excellent choice. These trowels allow you to apply the plaster smoothly while maintaining control over its thickness and texture.
Once the first coat is applied, it should be levelled using a straight edge or level to ensure a smooth, even surface. Here are the ideal tools for this task:
- Serrated H Section Feather Edges: These are highly popular for levelling freshly applied render, as they help remove air pockets and identify high and low spots.
- H Section Feather Edges: Offering enhanced grip compared to traditional feather edges, these are excellent for levelling sand and cement renders, as well as backing coats.
- Trapezoid Feather Edges: Lightweight and durable, these tools are perfect for sweeping across surfaces to detect high and low areas, ensuring an even finish on walls.
- Heavy-Duty Box Sections: Ideal for both wall and floor installations, these box sections help identify high and low spots, ensuring materials are perfectly level.
Step 5: Scratching Process & Second Coat
While the render is still wet, use a scarifier to scratch the first coat. This process enhances adhesion, making it easier for the second coat to bond securely to the surface. For the best results, allow the scratch coat to fully dry before applying the second and final coat.
The second coat of sand and cement render should be mixed at a ratio of 5 parts sand to 1 part cement. Once applied, follow the same process as in step 4, using a level to rule the surface flat. Remember to flatten the surface vertically and horizontally for the best results.
Key Features of Cement Plaster:
- Durability: Cement plaster creates a strong, long-lasting surface that can withstand harsh environmental conditions and daily wear.
- Water Resistance: It offers protection against moisture, making it suitable for humid areas, though it may need additional waterproofing for constant water exposure.
- Versatility: Can be used for both interior and exterior walls, offering flexibility in application and design.
- Fire Resistance: Cement plaster is non-combustible, providing a fire-resistant barrier for added safety in buildings.
- Adhesion: Bonds well to a variety of surfaces, including brick, concrete, and masonry, ensuring a stable and even finish.
- Smooth Finish: Provides a smooth, uniform surface that can be painted or finished with various decorative treatments.
- Low Maintenance: Once applied, cement plaster requires minimal maintenance, making it a cost-effective solution over time.
- Thermal Insulation: Offers some degree of thermal insulation, helping to maintain temperature stability in buildings.
- Sound Insulation: Helps reduce sound transmission, contributing to noise control in residential and commercial spaces.
Final Thoughts
Surface preparation is crucial for the success of any plastering project. Whether using gypsum, lime, or cement plaster, each material demands specific preparation techniques to ensure proper adhesion, durability, and a smooth finish. REFINA offers a comprehensive range of specialised tools, including scrapers, scarifiers, plastering trowels, and plastic trowels, equipping you with everything needed to achieve great results, every time.
Explore the full range or download our comprehensive catalogue to learn more about our premium range of plastering hand tools. By following the appropriate preparation steps and using high-quality tools, you can ensure that your plastering projects stand the test of time.