Drum sander dips at the start of a run are one of the most common mistakes DIYers make when sanding timber floors — and the good news is they're almost entirely preventable with the right technique. The dip happens when the rotating drum sits stationary (or nearly so) on the floor surface, cutting deeper into one spot before the machine moves forward. Master the correct start motion and you'll avoid this frustrating problem entirely.
Why the Dip Happens
A drum sander works by pressing a rotating abrasive drum against the floor. When you lower the drum while standing still, even for a fraction of a second, the drum grinds away at the same spot repeatedly, cutting a shallow hollow into the timber. The faster the drum spins and the coarser the grit, the worse the dip. On softer NZ timbers like rimu or heart rimu, this can happen almost instantly.
The Correct Start Technique
The golden rule is simple: the machine must always be moving forward before the drum touches the floor.
Step-by-step start motion
- Position yourself at the starting wall, with the sander tilted back slightly so the drum is raised off the floor.
- Start the motor and let it reach full speed — this usually takes a few seconds.
- Begin walking forward at a steady pace.
- Lower the drum smoothly as you move — not before.
- Maintain a consistent walking speed for the entire run. Slowing down mid-run can also cause shallow dips or uneven sanding.
- At the end of the run, raise the drum before you stop walking — the same principle applies in reverse.
Think of it like landing a plane: you want a smooth, moving contact, never a hard stop-and-drop.
Grit Selection Matters
Using too coarse a grit increases the risk of damage from any hesitation. A common approach for NZ hardwood floors (such as matai, rimu, or kauri) is:
| Pass | Grit | Purpose |
|---|---|---|
| 1st | 40–60 | Remove old finish, level boards |
| 2nd | 80 | Smooth out scratches from coarse pass |
| 3rd | 100–120 | Final smoothing before finishing |
If your floor only needs a light sand, start at 60 or 80 grit — there's no need to begin at 40 unless you're dealing with heavy paint, thick old varnish, or significantly cupped boards.
Walking Speed and Overlap
Consistent speed is just as important as the start technique. A good rule of thumb is a slow, deliberate walking pace — roughly the speed you'd push a lawnmower. Too slow and you risk burning or over-cutting; too fast and you won't remove enough material.
Overlap each run by about 75–100 mm to avoid ridges between passes. Keep your runs parallel to the timber grain wherever possible — sanding across the grain leaves scratches that are very difficult to remove.
Dealing with Existing Dips
If you've already sanded a dip into the floor, don't panic. Depending on the depth:
- Shallow dips (less than 1 mm): Subsequent passes with finer grits, feathering the area carefully with the drum sander or an edge sander, can often blend them out.
- Deeper dips: You may need to go back to a coarser grit and carefully re-sand the surrounding area to bring the level down to match. This takes patience and a good eye.
- Very deep gouges: In rare cases, a floor filler or timber putty matched to your species may be needed before the final fine-grit pass. Check products at Bunnings or Mitre 10 for suitable timber fillers.
Machine Hire Tips for NZ Homeowners
Most DIYers hire a drum sander rather than buy one. When hiring:
- Ask the hire shop to demonstrate the tilt-and-lower mechanism on the specific model — they vary between brands.
- Check that the drum is tracking straight and the abrasive belt is fitted correctly before you start.
- Hire an edge sander at the same time for the perimeter — the drum sander can't reach the edges.
- Wear hearing protection and a dust mask rated P2 or better. Older NZ homes (pre-1980s) may have lead-based paint or stains on floors — check with WorkSafe NZ for safe handling guidance before you start sanding.
Finishing Your Floor
Once sanding is complete, vacuum thoroughly and tack-cloth the surface before applying your chosen finish. Popular NZ options include water-based polyurethane (lower VOCs, faster drying) and oil-based finishes (harder wearing, longer dry time). Resene and Wattyl both offer floor finish ranges suited to NZ conditions. Always follow the manufacturer's recoat windows — applying too soon between coats is a common cause of finish failure.
When to Call a Professional
Drum sanding is a skill that takes practice, and mistakes can mean removing more timber than intended — a problem on older NZ homes where boards may already be thin from previous sandings. Consider hiring a qualified floor sanding professional if:
- Your floor has significant cupping, crowning, or movement between boards.
- You're dealing with exotic or very soft timbers that are easily damaged.
- The floor has existing deep dips or damage that need expert assessment.
- You're not confident with the machine after a test run on a hidden area.
A professional floor sander will have commercial-grade equipment and the experience to read the floor correctly. You can find qualified local floor sanding tradespeople on tradedirectory.co.nz.
