What Coat Before Plastering?

Preparation is the key to a smooth and long-lasting plaster finish. If you want your surface to be ready for the first coat of plaster, you must first apply the correct undercoat. The quality and durability of the plaster are both improved by this preparation process, which also promotes adhesion.

Learn about the many kinds of undercoats, when to use each one, and why they’re crucial to a smooth plastering project in this comprehensive guide. Whether you’re a weekend warrior or a seasoned builder, mastering the pre-coat is the first step to flawless results.

What Coat Before Plastering?

Before applying plaster to a wall, it’s essential to prepare the surface with a suitable undercoat to ensure proper adhesion and a smooth finish. This preparatory layer is often referred to as a “bonding coat” or “priming coat,” and it plays a crucial role in the overall success of the plastering process. Here are the key options for what to apply before plastering, get redirected here:

PVA (Polyvinyl Acetate) Bonding Agent

PVA is a popular choice for prepping surfaces before plastering. When diluted with water, it acts as a primer that helps the plaster adhere better to the substrate. It is particularly useful on porous surfaces, such as old plaster or masonry.

  • How to use: Mix PVA with water in a 1:4 ratio and apply it evenly to the wall. Allow it to become tacky before applying the plaster.

Bonding Plaster

Bonding plaster is a type of undercoat plaster designed to provide a strong base on difficult surfaces, such as smooth concrete or dense surfaces with low suction. It helps create a key for the finishing coat of plaster to adhere to.

  • How to use: Apply a thin, even layer of bonding plaster directly to the wall and allow it to set before adding the final plaster coat.

Blue Grit or Plastering Grit

This product is a gritty, adhesive primer designed to improve the adhesion of plaster to smooth, low-suction surfaces. It contains fine sand particles that create a textured surface for the plaster to grip onto.

  • How to use: Apply the blue grit to the surface with a brush or roller, ensuring an even coverage. Allow it to dry completely before plastering.

SBR (Styrene-Butadiene Rubber) Bonding Agent

SBR is a versatile bonding agent that enhances the adhesive properties of plaster on various substrates. It is particularly effective on surfaces exposed to moisture or subject to movement.

  • How to use: Mix SBR with water according to the manufacturer’s instructions and apply it to the wall. Wait for it to become tacky before applying the plaster.

Why Use A Pre-Coat?

Using a pre-coat before plastering offers several benefits:

  • Improves Adhesion: Ensures the plaster bonds well to the substrate, reducing the risk of peeling or cracking.
  • Regulates Suction: Helps control the absorption rate of the substrate, preventing the plaster from drying too quickly.
  • Creates a Uniform Surface: Provides a consistent base for the plaster, leading to a smoother and more even finish.
  • Enhances Durability: Increases the longevity of the plaster by providing a stable foundation.

When it comes to obtaining a high-quality finish, selecting the appropriate pre-coat before plastering is quite necessary. While dealing with PVA, bonding plaster, blue grit, or SBR, each substance works to improve the performance and durability of the plaster. This is true regardless of the type of plaster being used. To ensure that your walls are not only aesthetically beautiful but also structurally sound, proper preparation is the cornerstone of effective plastering.

How Do You Waterproof A Wall Before Plastering?

Waterproofing a wall before plastering is crucial in preventing moisture from penetrating and damaging the plaster. Here are the steps to effectively waterproof a wall before plastering:

Clean The Wall Surface

Remove Dirt and Debris: Start by cleaning the wall surface thoroughly. Remove any dirt, dust, loose paint, or old plaster. Use a stiff brush or a scraper to ensure the wall is as clean as possible.

Repair Cracks and Holes: Fill any cracks or holes with a suitable filler. Allow it to dry completely before proceeding.

Apply A Waterproofing Agent

Choose an appropriate waterproofing agent based on the type of wall and the degree of moisture exposure. Common waterproofing agents include:

  • Cementitious Waterproofing: This involves applying a waterproof cement-based coating that is easy to apply and effective for many masonry surfaces.

How to Use: Mix the cementitious waterproofing powder with water as per the manufacturer’s instructions. Apply the mixture with a brush or roller in an even layer. Allow it to dry completely before proceeding with the next step.

  • Liquid Waterproofing Membrane: A liquid membrane forms a flexible, waterproof layer when applied to the wall.

How to Use: Apply the liquid waterproofing membrane with a brush or roller, ensuring an even coverage. Usually, two coats are recommended. Allow the first coat to dry completely before applying the second.

  • Bituminous Coating: Bituminous coatings provide excellent waterproofing and are ideal for walls that are below ground level or exposed to significant moisture.

How to Use: Apply the bituminous coating with a brush or roller, ensuring thorough coverage. Allow it to dry completely, which may take up to 24 hours.

Apply A Primer/Bonding Agent

Once the waterproofing layer is dry, apply a primer or bonding agent to ensure the plaster adheres properly.

  • PVA (Polyvinyl Acetate) Primer: Mix PVA with water in a 1:4 ratio and apply it to the wall. Allow it to become tacky before plastering.
  • SBR (Styrene-Butadiene Rubber) Bonding Agent: Dilute SBR according to the manufacturer’s instructions and apply it to the wall. Wait until it becomes tacky before applying the plaster.

Reinforce With Mesh (if necessary)

In areas prone to cracking or where additional reinforcement is required, apply a fibreglass mesh over the waterproofing layer.

How to Use: Embed the mesh in a thin layer of the waterproofing agent or adhesive. Ensure it is smooth and free from wrinkles. Allow it to dry before proceeding with the plaster.


After ensuring the wall is properly waterproofed and primed, proceed with plastering.

  • Mix Plaster: Prepare the plaster mix according to the manufacturer’s instructions.
  • Apply Base Coat: Apply the base coat of plaster evenly using a trowel. Allow it to set slightly.
  • Finish Coat: Apply the finish coat to achieve a smooth surface. Use appropriate tools to ensure an even and level finish.

Waterproofing a wall before plastering involves cleaning the surface, applying a waterproofing agent, using a primer or bonding agent, and reinforcing with mesh if needed. Following these steps ensures that the plaster will adhere properly and remain durable, protecting the wall from moisture damage.


To make sure your plaster finish lasts and doesn’t wear out quickly, waterproof the wall before you apply the plaster. To get ready, you need to clean the area well, fix any damage to the surface, and then apply a waterproofing chemical. Coatings made of cement, liquid waterproofing membranes, or bituminous material all serve as effective barriers against water. The plaster will adhere better if you use a primer or bonding agent after this.

Fibreglass mesh can be used as an additional layer of reinforcement in crack-prone areas. A smooth, long-lasting plaster finish that is resistant to moisture may be yours with careful attention to detail and the following processes. Your walls will be protected and look great for years to come.