Applying a bonding agent to a smooth concrete wall before rendering is an essential step — without it, render is likely to delaminate or crack away from the surface over time. The good news is that the process is straightforward for a competent DIYer, as long as you prepare the surface properly and choose the right product for the job.
Why Smooth Concrete Needs a Bonding Agent
Ordinary render relies on a mechanical key — tiny surface irregularities that the render grips onto as it cures. Smooth, formed concrete (such as tilt-slab or poured concrete walls) has very little of this texture, making it a notoriously poor substrate for render adhesion. A bonding agent (sometimes called a bonding primer or slurry coat) chemically and mechanically bridges the gap between the concrete and the render coat, dramatically improving adhesion and reducing the risk of failure.
For exterior walls, good adhesion is especially important in New Zealand's variable climate — thermal movement, moisture cycling, and UV exposure all put stress on the render-to-substrate bond. Refer to BRANZ for guidance on render systems suited to NZ conditions.
What You'll Need
- Bonding agent (see product options below)
- Stiff-bristle brush or roller (as recommended by the product)
- Bucket and mixing paddle (if using a slurry-type product)
- Pressure washer or hose
- Safety glasses and gloves
- Drop sheets to protect surrounding areas
Step 1 — Prepare the Surface Thoroughly
This is the most critical step. Even the best bonding agent will fail on a dirty or contaminated surface.
- Remove all dust, dirt, oil, grease, and form-release agents. These are common on poured concrete and will prevent adhesion. Use a degreaser if needed, then rinse well.
- Pressure wash the wall and allow it to dry completely, or follow your product's instructions regarding surface moisture (some bonding agents require a damp surface, others require it to be dry).
- Check for laitance — the weak, powdery layer that sometimes forms on the surface of concrete. If present, it must be removed by grinding, scarifying, or acid etching before you apply any bonding agent.
- Repair any cracks or spalling with a suitable repair mortar before proceeding.
Step 2 — Choose the Right Bonding Agent
There are two main types used in NZ rendering:
| Type | How It Works | Typical Use |
|---|---|---|
| Acrylic bonding primer (liquid) | Brush or roll onto surface; render applied once tacky or dry | Most smooth concrete walls |
| SBR (Styrene Butadiene Rubber) slurry | Mixed with cement and sand; scrubbed onto surface | Difficult substrates, high-movement areas |
Well-known products available in NZ include Sika bonding agents (such as Sika bonding agent and primer ranges) and Ardex primers. Mapei also offers bonding solutions suitable for render preparation. Always check the product's technical data sheet (TDS) to confirm it is suitable for your specific render system — not all bonding agents are compatible with all renders.
You can find these products at Bunnings, Mitre 10, or PlaceMakers.
Step 3 — Apply the Bonding Agent
Acrylic Bonding Primer
- Stir or shake the product well before use.
- Apply evenly to the clean, prepared concrete surface using a brush or roller, working it into any surface texture.
- Allow to become tacky (or fully dry, depending on the product instructions) before applying render. This window is critical — applying render too early or too late can reduce effectiveness.
- In hot or windy conditions, the open time may be shorter, so work in manageable sections.
SBR Slurry Coat
- Mix SBR bonding agent with Portland cement and water to a thick, creamy consistency (follow the manufacturer's ratio — typically around 1 part SBR to 1 part water, mixed into cement).
- Scrub the slurry firmly onto the damp concrete surface using a stiff brush, ensuring full coverage.
- Apply your first render coat while the slurry is still green (not fully set) — this is the key to the slurry method working correctly.
Step 4 — Apply Your First Render Coat
Once the bonding agent is ready, apply your scratch coat (first render coat) firmly, pressing it into the bonding agent. Scratch the surface with a comb scratcher to provide a key for subsequent coats. Allow each coat to cure as per the render manufacturer's instructions before applying the next.
Building Code and Consent Considerations
Rendering a wall is generally exempt from building consent under Schedule 1 of the Building Act, but this depends on the scope of work and location. If the rendering is part of a weathertightness system on an exterior wall, or involves structural elements, check with your local council first. The MBIE Building Performance website is a good starting point for understanding consent requirements.
When to Call a Professional
While applying a bonding agent is within reach of a capable DIYer, consider hiring a qualified renderer if:
- The wall is large, high, or requires scaffolding (see WorkSafe NZ for working-at-height requirements)
- The concrete has significant laitance, contamination, or structural cracks
- You are rendering as part of an exterior weatherproofing system where failure could cause water ingress
- You are unsure which render system is compatible with your bonding agent
A licensed renderer will know which products work best together and can ensure the system meets NZ Building Code requirements. You can find qualified local renderers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
