Knotty floorboards can look stunning once finished, but they do require more preparation than clear-grained timber. The good news is that with the right approach — treating resinous knots, using a proper grit sequence, and filling any voids — you can achieve a beautifully smooth result that highlights the natural character of the wood.
Why Knots Cause Problems
Knots are denser than the surrounding timber and often contain resin (sap), which can bleed through finishes, cause adhesion failures, and leave sticky or discoloured patches. They also sand at a different rate to the surrounding wood, which can create an uneven surface if you rush through the grits. Understanding these two issues — resin bleed and uneven hardness — is the key to tackling knotty floors successfully.
Step 1: Assess and Prepare the Floor
Before you switch on any machinery, get down on your hands and knees and inspect every board.
- Check for loose or raised knots. Small loose knots (sometimes called "dead knots") can fall out during sanding, leaving a hole. Secure any that are wobbly with a quality wood adhesive and allow it to cure fully before sanding.
- Look for resin pockets. Sticky, amber-coloured patches indicate active resin. These need to be treated before finishing (see Step 2).
- Punch all nail heads at least 3 mm below the surface so they don't tear your sanding belts.
- Check for movement or squeaks and fix any loose boards by face-nailing or screwing them down.
Step 2: Treat Resinous Knots
This is the step most DIYers skip — and it's the one that causes the most grief later. Resin will prevent your stain or finish from bonding properly and can cause yellowing or tackiness.
- Clean the resin from the surface using a rag dampened with methylated spirits or a dedicated resin solvent. Work in a well-ventilated area.
- Once clean and dry, apply a shellac-based primer or knotting solution (sometimes called "knotting compound") over each affected knot. This seals the resin in and gives your finish something to bond to. Products like Zinsser BIN (shellac-based) are widely available at Bunnings and Mitre 10.
- Allow the knotting solution to dry completely — usually 30–45 minutes — before sanding.
Step 3: Sand in the Correct Grit Sequence
A drum sander (for the main floor area) and an edge sander are the standard tools for floor sanding. Hire these from your local hire centre if you don't own them.
Recommended grit sequence for knotty floors
| Pass | Grit | Purpose |
|---|---|---|
| 1st | 40–60 grit | Remove old finish, level the floor |
| 2nd | 80 grit | Remove scratches from coarse pass |
| 3rd | 100–120 grit | Smooth the surface |
| Final | 120–150 grit | Fine finish before coating |
- Don't skip grits. Jumping from 40 to 120 grit leaves deep scratches that show under a clear finish.
- Sand with the grain wherever possible. Diagonal or cross-grain sanding can be used on the first pass if the floor is very uneven, but always finish along the grain.
- Go slowly over knots. Because knots are harder, the surrounding softer wood sands away faster. Use a light touch and keep the machine moving to avoid dips forming around each knot.
- Use a hand scraper or detail sander in tight corners and around knots where the drum sander can't reach cleanly.
Step 4: Fill Voids and Gaps
After your second or third sanding pass, you'll have a good amount of fine sawdust. Mix this with a compatible wood filler or gap filler to create a slurry that matches your floor colour closely. Products from Sika or Ardex offer flexible fillers suitable for timber floors. Fill any knot voids, cracks, or gaps between boards, allow to cure, then sand flush on your final pass.
Tip: Avoid rigid fillers in gaps between boards — NZ's humidity swings mean timber expands and contracts seasonally, and a rigid filler will crack or pop out.
Step 5: Vacuum and Tack Off
Before applying any finish, vacuum the entire floor thoroughly, then wipe it down with a tack cloth or a slightly damp microfibre cloth to remove all fine dust. Any dust left on the surface will be locked into your finish and create a rough, gritty texture.
Step 6: Apply Your Finish
For knotty floors, a water-based polyurethane is a popular choice — it's low-odour, dries quickly, and doesn't yellow over time (which can make knots look muddy). Oil-based polyurethanes give a warmer amber tone but take longer to dry and have stronger fumes.
- Apply a sealer coat first, which soaks into the timber and provides a stable base.
- Lightly sand between coats with 220-grit paper or a fine abrasive pad to knock back any raised grain.
- Apply two to three topcoats, allowing full drying time between each.
Resene, Dulux, and Wattyl all offer floor-specific coating systems with technical datasheets to guide your application.
When to Call a Professional
Floor sanding looks straightforward but is genuinely skilled work. Drum sanders are powerful machines — a moment's inattention can gouge deep tracks that are very difficult to repair. Consider hiring a professional floor sander if:
- Your floor has many large or loose knots that need careful individual attention.
- The boards are thin (under about 18 mm) and at risk of being sanded through.
- You want a stained or specialist finish where colour consistency over knots is critical.
- You've attempted a pass and the floor looks uneven.
A qualified floor sanding professional will have the right equipment, experience with knotty NZ timbers like rimu and pine, and access to commercial-grade finishes. You can find experienced local floor sanders on tradedirectory.co.nz.
Safety note: Sanding dust is a respiratory hazard. Always wear a P2 dust mask, eye protection, and hearing protection. Ensure good ventilation. Finishing products can be flammable — keep ignition sources away and follow the manufacturer's safety instructions. Check WorkSafe NZ for guidance on safe use of solvents and dust management.
