Getting your surface prep right is the single biggest factor in how good your paint job looks and how long it lasts. Sandpaper grit is a key part of that prep — use too coarse a grit and you'll leave scratches that show through the paint; too fine and the surface won't have enough tooth for the paint to grip. Here's a straightforward guide to choosing the right grit for the job.
Understanding Grit Numbers
Grit refers to the number of abrasive particles per square inch on the sandpaper. The lower the number, the coarser the paper and the more aggressively it cuts. Higher numbers mean finer paper, which smooths rather than removes material.
As a general rule:
- 40–80 grit — coarse, for heavy material removal
- 100–150 grit — medium, for general sanding and shaping
- 180–220 grit — fine, for final prep before painting
- 240–320 grit — very fine, for sanding between coats
- 400+ grit — ultra-fine, for final flatting on high-gloss finishes
Sanding Bare Timber
New or bare timber is one of the most common surfaces NZ homeowners paint, whether it's weatherboards, decking, window joinery, or interior trim.
- Start with 80–120 grit to remove mill glaze, rough grain, or old filler.
- Follow up with 180–220 grit to smooth the surface before priming.
- Always sand with the grain to avoid cross-grain scratches that show under paint.
For rough-sawn timber or heavily weathered wood, you may need to start as coarse as 60 grit before working up through the grits. BRANZ recommends thorough surface preparation as the foundation of any durable paint system.
Sanding Plasterboard (GIB)
If you're painting new GIB plasterboard or freshly stopped walls, the approach is different. The paper face of plasterboard is delicate — too coarse a grit will scuff and raise the surface.
- Use 120–150 grit to lightly sand stopping compound (Gib Stop or similar) once it's fully dry.
- Finish with 180–220 grit for a smooth, flat surface ready for primer.
- Avoid over-sanding — you only want to knock back ridges and trowel marks, not dig into the board itself.
Repainting Over Existing Paint
When repainting walls, ceilings, or exterior surfaces that already have paint on them, the goal is to degloss and key the surface so the new coat bonds properly.
- For low-sheen or flat paints, a light sand with 180–220 grit is usually enough.
- For semi-gloss or gloss finishes (common on NZ kitchens, bathrooms, and trim), use 120–150 grit to properly break the sheen, then finish with 180–220 grit.
- Spot-sand any flaking, bubbling, or peeling areas back to a firm edge before filling.
Resene and Dulux both publish surface preparation guides that recommend keying glossy surfaces before recoating — it's worth checking the specific product datasheet for the paint you're using.
Sanding Between Coats
For a truly professional finish — especially on doors, trim, and cabinetry — lightly sand between every coat.
- Use 220–280 grit between primer and topcoat.
- Use 320–400 grit between topcoats on high-gloss finishes.
- Always remove all dust with a tack cloth or damp rag before applying the next coat.
This step is what separates a flat, smooth finish from one that looks bumpy or has visible brush marks.
Choosing the Right Sandpaper Type
Not all sandpaper is the same. Here's a quick comparison of common types available at NZ hardware stores:
| Type | Best For | Notes |
|---|---|---|
| Aluminium oxide | Timber, paint, primer | Durable, good all-rounder |
| Silicon carbide | Between coats, plaster | Good for fine finishing |
| Garnet | Bare softwood timber | Traditional, less aggressive |
| Sanding sponges | Curved surfaces, mouldings | Easy to use, flexible |
| Mesh/screen sheets | Plasterboard stopping | Resists clogging |
You'll find a wide range of sandpaper and sanding accessories at Mitre 10 and Bunnings.
Tools: Hand Sanding vs Power Sanders
- Hand sanding with a cork or foam block gives you good control for small areas, trim, and detail work.
- Random orbital sanders are ideal for large flat surfaces like walls, floors, and exterior cladding — they leave fewer swirl marks than belt sanders.
- Pole sanders are great for ceilings and large wall areas.
For interior sanding, especially on plasterboard, consider a dustless sanding system with a vacuum attachment — plaster dust is very fine and gets everywhere.
Health & Safety When Sanding
Sanding generates fine dust that can be harmful to your lungs. Always:
- Wear a P2-rated dust mask (not just a paper dust mask).
- Ensure good ventilation.
- If sanding paint on a home built before the 1980s, be aware that lead-based paint may be present. Check with WorkSafe NZ for guidance on safe handling before you start.
When to Call a Professional
Most sanding and surface prep is well within a capable homeowner's ability. However, consider hiring a qualified painter if:
- You're dealing with lead paint on an older home.
- The surface has extensive damage, rot, or moisture issues that need specialist assessment.
- You want a high-end finish on feature joinery, cabinetry, or a full exterior repaint.
A professional painter will have the right tools, products, and experience to get a lasting result. You can find qualified local painters on tradedirectory.co.nz.