Finishing an internal corner correctly is one of the most important skills in plastering — done right, it gives you a crisp, crack-free line that will last for years. The correct method involves embedding paper tape into a bed of stopping compound, building up thin coats, and feathering the edges out smoothly. Rushing any step is the most common cause of cracking and ugly ridges.
What You'll Need
Before you start, gather the following:
- Paper jointing tape (not fibreglass mesh, which is not recommended for internal corners)
- Stopping compound (also called jointing compound or plaster stop) — pre-mixed or powder-based
- Corner trowel (an internal corner trowel makes life much easier)
- Broad flat trowel or plasterer's knife (150–250 mm)
- Sanding block and fine-grit sandpaper (120–180 grit)
- Primer before painting
For NZ plasterboard work, always follow the system recommended by your board manufacturer. GIB publish detailed stopping and finishing guides specific to their products, and these are the industry standard reference for most NZ residential work.
Step 1 — Prepare the Corner
Check that both plasterboard sheets meet cleanly at the corner with no gaps larger than about 3 mm. If the gap is wider, pack it or use a setting-type compound to fill it first and let it cure fully before proceeding. Remove any dust, loose paper facing, or debris. The surface must be clean and dry.
Step 2 — Apply the First Bed Coat
Using your trowel or broad knife, apply a thin, even coat of stopping compound into the corner, covering both faces of the angle. Aim for a coat roughly 1–2 mm thick and about 75–100 mm wide on each side of the corner. Work in manageable lengths — around 600–900 mm at a time if you're learning.
Step 3 — Embed the Paper Tape
This is the critical step. Paper tape is essential for internal corners — it reinforces the joint and prevents cracking as the building moves slightly over time.
- Fold the paper tape along its pre-creased centre line.
- Press it firmly into the wet compound in the corner, working from top to bottom.
- Use your corner trowel or the edge of your flat trowel to bed the tape in firmly, squeezing out air bubbles and excess compound from beneath.
- Wipe away any excess compound that squeezes out, leaving the tape flat and fully embedded.
Allow this coat to dry completely — typically 24 hours, though this varies with humidity and temperature. NZ's variable climate means drying times can be longer in winter or in poorly ventilated rooms.
Step 4 — Second Coat
Once fully dry, apply a second, slightly wider coat of compound over the tape on both faces of the corner. Feather the edges out to around 150–200 mm from the corner on each side. Use long, smooth strokes and keep the coat thin — thick coats shrink and crack. A corner trowel helps you keep both sides even in one pass.
Let this coat dry fully before moving on.
Step 5 — Third (Finish) Coat
Apply a final thin skim coat, feathering even further out — up to 200–250 mm each side. The goal is a perfectly flat surface with no ridges or tool marks. Some plasterers use a slightly diluted compound for this coat to improve workability and reduce drag marks.
Step 6 — Sand and Prime
Once fully cured, lightly sand the area with 120–180 grit sandpaper on a block. Do not over-sand — you can easily damage the paper face of the plasterboard. Wipe away dust with a damp cloth and allow to dry.
Always prime before painting. A quality sealer/primer is essential to prevent the stopped areas from flashing (showing through the topcoat). Resene, Dulux, and Wattyl all offer suitable interior primers — check the product data sheet for the recommended system.
Common Mistakes to Avoid
| Mistake | Why It's a Problem | Fix |
|---|---|---|
| Using mesh tape in corners | Mesh doesn't fold cleanly; prone to cracking | Always use paper tape for internal corners |
| Applying coats too thick | Compound shrinks as it dries, causing cracks | Multiple thin coats are always better |
| Not feathering edges | Creates visible ridges under paint | Widen each coat progressively |
| Painting without priming | Stopped areas absorb paint unevenly (flashing) | Always prime first |
| Rushing drying time | Trapping moisture causes bubbling and cracking | Allow full drying between coats |
NZ Building Code Context
Internal plastering and stopping is generally not restricted work under the NZ Building Code, meaning a competent homeowner can carry out their own stopping and finishing. However, if you're doing significant alterations or additions, a building consent may be required — always check with your local council. The MBIE Building Performance website is a good starting point for understanding consent requirements and Schedule 1 exemptions.
For fire-rated wall systems (common in multi-unit dwellings or between a garage and living space), the stopping system must comply with the specific tested assembly — follow the GIB or manufacturer's specification exactly and don't substitute products.
When to Call a Professional
While stopping internal corners is a DIY-friendly task, there are situations where it pays to bring in a qualified plasterer:
- Large areas or whole-house stopping — consistency and speed matter, and an experienced plasterer will achieve a far better finish faster.
- Textured or skim-coat finishes — these require significant skill and practice.
- Recurring cracks — if corners keep cracking after repair, there may be a structural movement issue that needs investigation.
- Fire-rated assemblies — must be completed to specification.
You can find qualified, local plasterers on tradedirectory.co.nz who are familiar with NZ conditions and products.
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