Ridges at the edges of stopping coats are one of the most common complaints from DIYers and even less-experienced tradespeople tackling plasterboard finishing. The good news is that ridges are almost always caused by technique rather than materials — so with a few adjustments, you can achieve a flat, paint-ready surface. Here's what you need to know.
Why Ridges Form in the First Place
A ridge forms when the edge of your stopping knife lifts and deposits a bead of compound rather than blending it smoothly into the surrounding surface. The main culprits are:
- Too much compound on the knife — excess material piles up at the trailing edge
- Holding the knife at too steep an angle — the blade digs in rather than gliding
- Stopping abruptly — ending a stroke without gradually lifting the blade
- Using a knife that's too narrow — a narrow blade can't spread compound wide enough to feather properly
- Compound that's too thick — stiff mix drags and ridges instead of flowing flat
The Key Technique: Feathering
Feathering is the process of gradually reducing the thickness of the compound toward the outer edge of each coat so it tapers to almost nothing. This is the single most important skill in ridge-free stopping.
To feather correctly:
- Load your knife with a modest amount of compound — less than you think you need.
- Apply the compound with a firm, even stroke, keeping the blade at a low angle (roughly 15–25° to the surface).
- As you near the end of the stroke, gradually reduce pressure on the back edge of the knife and lift the leading edge slightly, so the compound thins out to a feather edge.
- On the return stroke, use a nearly clean knife held at a very low angle to skim off any ridge left behind.
- Wipe your knife clean between strokes — dried bits of compound dragged across wet compound are a major ridge-maker.
Choose the Right Tools
Tool selection makes a real difference:
- Wide finishing knives (200–300 mm) are essential for feathering. Narrow knives can't span the compound edge and the surrounding board, so they always leave a ridge.
- Flexible blades — a slightly flexible stainless steel blade conforms to minor surface variations and reduces drag.
- Mud pans — using a hawk or mud pan rather than working straight from the bucket helps you control the amount of compound on the knife.
You'll find a good range of plastering knives and tools at Bunnings and Mitre 10, and PlaceMakers stocks trade-grade options.
Get Your Compound Consistency Right
Pre-mixed stopping compounds should generally be used straight from the bucket, but if the mix has dried out slightly, add a very small amount of water and stir thoroughly. The compound should be smooth and creamy — similar to thick yoghurt. If it's stiff and lumpy, it will drag and ridge no matter how good your technique is.
For GIB board systems, always use a compound recommended for the specific coat (base coat, finishing coat). Using a base coat compound for a finish coat, for example, can make feathering much harder because the mix is coarser.
Apply Multiple Thin Coats
One of the biggest DIY mistakes is trying to fill everything in one thick coat. Thick coats shrink unevenly as they dry, and they're almost impossible to feather. Instead:
| Coat | Purpose | Typical thickness |
|---|---|---|
| First (base) coat | Fill the tape and scrim, cover fasteners | 1.5–2 mm |
| Second coat | Widen and flatten the first coat | 0.5–1 mm |
| Third (finish) coat | Final skim, feathered wide | As thin as possible |
Each coat should extend 50–75 mm wider than the previous one. This progressive widening is what creates a gradual, invisible taper rather than a visible ridge.
Sand Carefully Between Coats
Once each coat is fully dry, lightly sand any ridges or high spots with 120-grit sandpaper before applying the next coat. Don't over-sand — you're just knocking off high points, not removing the whole coat. Wipe away dust before recoating.
Always wear a dust mask when sanding stopping compound. WorkSafe NZ advises that fine plaster dust can be harmful with repeated exposure.
Common Mistakes to Avoid
- Overloading the knife — start with less compound than you think you need
- Rushing — let each coat dry fully before the next (check the manufacturer's data sheet for drying times)
- Stopping mid-stroke — always complete the full stroke before lifting the knife
- Dirty tools — rinse knives and pans thoroughly between sessions; dried compound ruins your edge
- Working in direct sun or wind — rapid drying causes the edges to crust before you can feather them; work in stable, moderate conditions where possible
When to Call a Professional
Stopping and plastering is not restricted work under NZ law, so homeowners can tackle it themselves. However, if you're working on a large area, a new build, or need a Level 5 finish for gloss paint, the margin for error is very small. A qualified plasterer will achieve a flatter result faster and with less material waste.
If your ridging problem is related to structural cracking, movement joints, or moisture damage to the plasterboard substrate, it's worth getting a professional assessment before you stop over the top — you may be covering up a deeper issue. You can find qualified local plasterers on tradedirectory.co.nz.
For guidance on what finishing standards apply to your project, BRANZ publishes detailed technical information on plasterboard finishing levels and acceptable tolerances.
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