Internal corner cracks are one of the most common frustrations in plastering and stopping work — but they're almost always preventable. The good news is that with the right materials, technique, and patience between coats, you can achieve corners that stay tight for years. Here's what NZ tradespeople know that makes the difference.
Why Internal Corners Crack
Before fixing the problem, it helps to understand the cause. Internal corners crack for a few main reasons:
- Structural movement — NZ homes, particularly timber-framed houses built to NZS 3604, experience seasonal timber shrinkage and normal building movement. Corners are stress points where two planes meet, making them vulnerable.
- Poor tape embedment — If the paper tape isn't fully bedded into wet compound, air pockets form and the tape lifts or cracks as it dries.
- Rushing between coats — Applying the next coat before the previous one is fully dry traps moisture and leads to shrinkage cracking.
- Using the wrong compound — Not all stopping compounds are equal. Using a setting-type compound for the base coat and a lightweight compound for finishing coats gives better results than using one product throughout.
- Insufficient compound thickness — Skimping on the base coat leaves the tape unsupported.
Choosing the Right Materials
For internal corners, you'll need:
- Paper tape — Do not use fibreglass mesh tape on internal corners. Paper tape is flexible and designed to fold cleanly into a corner. Mesh tape is too rigid and prone to cracking in corners.
- Setting compound (base coat) — A powder-based setting compound (sometimes called "hot mud") for the first coat gives a hard, stable base. GIB and other suppliers offer suitable products through merchants like PlaceMakers and Mitre 10.
- Lightweight finishing compound — For second and third coats, a pre-mixed lightweight compound sands back easily and reduces shrinkage.
- Corner tool or flexible knife — A dedicated internal corner tool or a pair of 100 mm and 150 mm flexible knives makes it much easier to get a clean, even angle.
Step-by-Step: Taping Internal Corners Correctly
1. Prepare the Surface
Make sure the plasterboard sheets are properly fixed with no loose boards or fasteners. Any movement in the substrate will telegraph straight through to the finished surface. Check that sheets meet neatly at the corner without large gaps.
2. Apply the Base Coat
Mix your setting compound to a smooth, lump-free consistency. Apply a generous coat of compound into the corner on both faces — enough to fully embed the tape without it floating on the surface.
3. Embed the Paper Tape
Fold the paper tape along its pre-creased centre line and press it firmly into the wet compound. Use your corner tool or knife to work from the centre outward, squeezing out air bubbles and excess compound. The tape should be fully wet and flat with no dry patches or bubbles. Apply a thin skim coat over the tape to cover it completely.
4. Let It Dry Completely
This is where most DIYers go wrong — do not rush. Allow each coat to dry fully before applying the next. In a well-ventilated NZ home this typically takes 24 hours per coat, but in cooler or more humid conditions (common in many parts of New Zealand) it can take longer. The compound should be uniformly white and hard to the touch, not cool or translucent.
5. Apply Second and Third Coats
Feather each subsequent coat out wider than the last — 150 mm, then 200–250 mm either side of the corner. Keep coats thin. Sand lightly between coats with 120-grit paper to knock back any ridges before the final coat.
6. Sand and Prime Before Painting
Once fully dry, sand smooth with 150–180 grit. Always apply a quality primer before painting — skipping primer is a common cause of visible cracking and poor paint adhesion. Resene and Dulux both offer suitable interior primers for stopped surfaces.
Dealing with Movement-Prone Corners
In homes with significant seasonal movement — especially older villas or homes with green timber framing — consider using flexible acrylic caulk instead of stopping compound in the very corner line itself. Apply paper tape and compound to both faces as normal, but leave a hairline gap at the apex of the corner and fill it with paintable flexible caulk once the stopping is complete and primed. This allows the joint to flex slightly without cracking. BRANZ has useful guidance on managing building movement in residential construction.
Common Mistakes to Avoid
| Mistake | Why It Causes Cracking |
|---|---|
| Using mesh tape on internal corners | Too rigid; cracks under movement |
| Rushing between coats | Trapped moisture causes shrinkage |
| Thin or uneven base coat | Tape lifts and cracks |
| Skipping primer before paint | Paint pulls moisture unevenly, stressing the joint |
| Painting in cold or damp conditions | Slows drying, increases movement |
When to Call a Professional
If your corners keep cracking despite correct technique, the problem may be structural — loose framing, inadequate nogging, or excessive building movement. This is worth having assessed by a qualified plasterer or Licensed Building Practitioner. Persistent cracking in specific areas can also indicate moisture issues behind the wall, which should be investigated before re-stopping. Plastering is not restricted trade work, but getting it right takes practice — a skilled plasterer will save you time and frustration. You can find qualified local plasterers on tradedirectory.co.nz.
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