Getting a new render coat to bond properly to an old smooth surface is one of the most important steps in any rendering job. Without adequate keying, the new coat is likely to delaminate, crack, or fail prematurely — no matter how good the render mix is. The good news is that with the right preparation, even a very smooth existing surface can be made ready for a durable new coat.
Why Keying Matters
Render relies on mechanical adhesion — tiny interlocking points between the new coat and the substrate. A smooth, dense surface offers almost no mechanical grip, so the new render has nothing to hold onto. Keying creates the surface profile needed for a strong bond. This is especially important in New Zealand's variable climate, where thermal movement and moisture cycling put real stress on render systems. Following good practice also helps you meet the intent of NZ Building Code clause E2 (External Moisture), which requires cladding systems — including render — to perform durably over time.
Step 1 — Assess the Existing Render
Before you do anything, check whether the old render is sound and well-bonded. Tap across the surface with a timber mallet or the handle of a screwdriver and listen for a hollow "drum" sound — this indicates delamination. Any hollow, cracked, or friable sections must be cut out and patched before you attempt to key the rest. Applying a new coat over failing render is a waste of time and money.
Also check for:
- Efflorescence (white salt deposits) — brush off and treat before proceeding
- Mould or algae — clean with a diluted bleach solution and allow to dry fully
- Contamination (oil, paint, form-release agents) — degrease thoroughly
Step 2 — Choose Your Keying Method
There are two main approaches: mechanical keying and chemical/bonding agent keying. In practice, combining both gives the best result.
Mechanical Keying
This is the most reliable method and should always be your first step on a smooth surface.
- Angle grinder with a scarifying or diamond cup wheel — fast and effective on hard cement render; creates a consistent scratch profile across large areas
- SDS hammer drill with a chisel or scabbling attachment — good for localised areas or very hard surfaces
- Hand-held scratcher/devil float — suitable for smaller areas or softer existing render; drag firmly across the surface in a cross-hatch pattern
- Wire brush or coarse abrasive disc — useful for lightly keying softer or older lime-based renders
Aim for a scratch depth of at least 2–3 mm across the whole surface. After mechanical keying, blow or brush off all dust and debris, then wash the surface down with clean water and allow it to dry to a saturated surface dry (SSD) condition before applying render.
Chemical Bonding Agents
A bonding agent or primer applied after mechanical keying significantly improves adhesion, particularly on dense or painted surfaces. Well-known products available in NZ include:
- Sika SikaBond or Sika render primers — check the current product range for render-specific options
- Ardex bonding primers — widely used in NZ trade work
- Mapei Planicrete or similar acrylic bonding agents — can also be added to the render gauging water
Always follow the manufacturer's data sheet exactly — application rates, open times, and compatibility with your render system vary between products.
Step 3 — Apply a Scratch Coat
On very smooth or difficult surfaces, consider applying a scratch coat (also called a spatterdash or slurry coat) before your main render coat. This is a thin, wet mix — often cement, sand, and a bonding agent — thrown or brushed onto the surface to build up a rough key. Once it has set (but not fully cured), scratch it with a comb scratcher to create a profile for the next coat. Allow it to cure for at least 24–48 hours before applying the main render.
Step 4 — Render System Compatibility
Make sure your new render is compatible with the existing substrate. As a general rule, never apply a stronger (higher cement content) render over a weaker one — the stronger coat will shrink and pull away. If the existing render is a lime-based or lightweight mix, use a similar or slightly weaker new coat. If you are unsure, consult the render manufacturer's technical data sheet or contact BRANZ for guidance on render system compatibility.
Safety Notes
Mechanical scarifying creates significant dust, which may contain crystalline silica — a serious respiratory hazard. Always wear a P2-rated dust mask or respirator, safety glasses, and hearing protection when using power tools. Check WorkSafe NZ guidelines on working safely with silica dust.
When to Call a Professional
While keying and rendering is not restricted work under NZ law, it is a skilled trade. Poor preparation or an incompatible render system can lead to costly failures — especially on external walls where moisture ingress can cause significant damage. If the existing render is failing over a large area, if the substrate is unknown, or if the wall forms part of a weathertight cladding system, it is well worth engaging a qualified renderer. You can find experienced local renderers on tradedirectory.co.nz. For work that may affect the building envelope, check with your local council whether a building consent is required — some recladding or remediation work does require consent even if straightforward repairs may fall under Schedule 1 exemptions.
Licensed Building Practitioner (LBP)
