Shrinking and cracking in stopping compound is one of the most common frustrations for DIYers tackling plasterboard finishing. The good news is that it's almost always preventable — and the causes come down to a handful of easily corrected mistakes around coat thickness, product choice, and drying conditions.
The Main Reason: Coats Applied Too Thick
The single biggest cause of cracking stopping compound is applying it too thickly in one go. Water-based stopping compounds (the most common type used over GIB plasterboard in NZ homes) shrink as the water evaporates during drying. The thicker the coat, the more water there is to escape — and the more the compound pulls inward as it dries, causing surface cracks or even deep splits.
The golden rule: apply multiple thin coats rather than one heavy coat. As a general guide:
- First (base) coat: fill the recess or gap, keeping it as flat as possible
- Second coat: skim over the first once fully dry to level and blend
- Third (finish) coat: a very thin skim for a smooth, paint-ready surface
Each coat should be allowed to dry completely before the next is applied. Rushing this process is a recipe for cracking.
Wrong Product for the Job
Not all stopping compounds are the same. There are two broad categories available at NZ suppliers like Bunnings and Mitre 10:
| Type | How it sets | Best use |
|---|---|---|
| Air-dry (water-based) compound | Dries by evaporation | General stopping, finishing coats |
| Setting-type (chemical-set) compound | Sets by chemical reaction | Deep fills, first coats, faster turnaround |
Using a standard air-dry compound to fill a deep gap (say, more than 6–8 mm) in a single pass is asking for trouble. For deeper fills, use a setting-type compound (sometimes labelled as "easy sand" or "hot mud") for the base coat, then finish with a lighter air-dry compound on top. Setting compounds shrink far less because they harden chemically rather than purely by water loss.
Also check the product hasn't passed its use-by date or been contaminated — old or lumpy compound can behave unpredictably.
Drying Too Fast: Heat, Wind, and Direct Sunlight
If compound dries too quickly on the surface while the interior is still wet, the outer skin tightens and cracks. This is especially common in NZ summers or in rooms with:
- Direct sunlight hitting the wall
- Heat pumps or fans blowing directly onto the work
- Poor ventilation causing uneven drying
Aim for slow, even drying at room temperature (roughly 15–25°C). Close blinds to avoid direct sun on fresh work, and don't point a heat pump directly at a freshly stopped wall. In cold, damp conditions (common in NZ winters), drying will be slower — be patient and ensure some background ventilation to prevent the compound staying wet for too long, which can cause mould issues.
Insufficient Surface Preparation
Stopping compound needs a clean, dust-free, and correctly primed surface to bond well. Common prep mistakes include:
- Not taping joints: Butt joints and internal angles between plasterboard sheets must be taped with paper or fibreglass mesh tape before stopping. Skipping tape means the compound has no reinforcement and will crack along the joint line.
- Skipping primer: Bare plasterboard paper face is very absorbent. Without a plasterboard primer (sometimes called a sealer or stopping primer), the board can suck moisture out of the compound too quickly, causing surface cracking. Check the GIB finishing systems guide for recommended primers.
- Dusty or oily surfaces: These prevent proper adhesion.
Mixing Consistency
If you're using a powder-based setting compound, mixing it too wet or too dry affects performance. Too much water increases shrinkage; too little makes it hard to work and prone to cracking as it can't flow into voids properly. Follow the manufacturer's mixing instructions carefully.
Pre-mixed compounds should be stirred before use if they've been sitting — but avoid whipping in excess air, which can create bubbles that weaken the dried surface.
Fixing Existing Cracks
If you already have cracks, don't just fill over them with another thick coat. Instead:
- Allow the area to dry completely
- Scrape back any loose or hollow-sounding material
- For joint cracks, re-tape if necessary
- Apply thin coats, feathering edges wide (200–300 mm either side of the crack)
- Sand lightly between coats once dry
For hairline cracks in paint rather than the stopping layer itself, a flexible interior filler or a quality paint like those from Resene or Dulux with good gap-filling properties may be sufficient.
When to Call a Professional
If you're finding that cracks keep returning in the same spots despite correct technique, it may indicate a structural movement issue — for example, timber framing drying out in a new build, or settlement in an older home. This warrants a closer look by a qualified plasterer or, if structural movement is suspected, a Licensed Building Practitioner (LBP).
Large-scale stopping and plastering work — particularly on new builds or significant renovations — is best left to a professional plasterer to ensure it meets the NZ Building Code requirements. You can find qualified local plasterers on tradedirectory.co.nz.
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