Knowing when to move to a finer sanding grit is one of the most important judgement calls in floor sanding — get it wrong and you'll either waste time and sandpaper, or end up with scratches locked under your finish. The golden rule is simple: only move to a finer grit once the current grit has completely removed every scratch left by the previous, coarser grit. Rush this step and those hidden scratches will show up clearly once the stain or coating goes on.
Understanding the Grit Sequence
Floor sanding typically follows a progressive grit sequence, moving from coarse to fine. A common sequence for a rough or previously coated floor might be:
| Stage | Grit Range | Purpose |
|---|---|---|
| Initial cut | 24–40 grit | Remove old finish, level boards |
| Intermediate | 60–80 grit | Remove coarse scratches |
| Fine | 100–120 grit | Smooth the surface |
| Final | 150–180 grit | Ready for coating |
Not every floor needs every stage — a lightly worn floor in good condition might start at 60 or 80 grit. If you're unsure where to begin, a qualified floor sander can assess your specific boards.
The Key Signs You're Ready to Move Up
1. No Visible Scratches from the Previous Grit
Get down low and use a raking light — a torch or work light held at a low angle across the floor surface. This is the most reliable method. If you can still see the deep, parallel scratch marks left by the coarser grit, you are not ready to move up. The current grit should have replaced those marks with its own finer, more uniform scratch pattern.
2. Consistent Colour Across the Whole Floor
When a coarser grit has done its job, the floor should look even in tone across the entire sanded area. Darker patches, shiny spots, or areas that look different from the rest usually mean the coarse grit hasn't fully cut through the old finish or stain in those spots. Sand those areas again before progressing.
3. The Scratch Pattern is Uniform
Run your hand lightly across the floor (carefully — splinters are possible). The surface should feel consistently textured, not patchy. Under raking light, the scratch marks should all run in the same direction and be the same depth. Random deep gouges or cross-hatching means more work is needed at the current grit.
4. You've Covered the Whole Area Properly
It sounds obvious, but make sure you haven't missed edges, corners, or areas near walls. The drum or belt sander handles the main field, but the edge sander needs to match it. Both areas must be at the same grit stage before you move on — mismatched grits between the field and edges are a very common cause of visible lines in the finished floor.
Common Mistakes to Avoid
- Skipping grits: Jumping from 40 grit straight to 120 grit leaves deep scratches that the finer paper can't remove efficiently. Follow the sequence.
- Moving up too soon: The most common error. Always check with a raking light before changing paper.
- Sanding against the grain: Always sand with the grain of the timber to avoid cross-grain scratches that are difficult to remove.
- Worn-out sandpaper: Dull paper burnishes the wood rather than cutting it, leaving a glazed surface that won't absorb finish evenly. Change paper regularly — it's false economy not to.
- Ignoring the edges: The edge sander scratches in a circular pattern. These circular marks must be sanded out with progressively finer grits on the edge sander, then hand-sanded along the grain before finishing.
NZ Timber Considerations
New Zealand homes commonly feature rimu, matai, heart kauri, and pine floors, each with different hardness and grain characteristics. Softer timbers like pine can be scratched more easily and may need a gentler starting grit. Native timbers like rimu and kauri are often irreplaceable, so take extra care — if in doubt, start with a less aggressive grit and test in a hidden area first. BRANZ has useful guidance on working with NZ timber species.
If your floor has been previously coated with a hard two-part polyurethane or an oil-based finish, the coarser grits will load up (clog) faster. Check and change your paper more frequently in these situations.
Dust and Safety
Floor sanding produces significant fine dust, which is a health hazard. Always wear a P2-rated dust mask, eye protection, and hearing protection. Ensure good ventilation. Some older NZ homes (pre-1980s) may have lead-based paint on skirting boards or subfloor coatings — if you suspect lead, check with WorkSafe NZ before proceeding. Hire a dust-extraction sander where possible to minimise airborne particles.
When to Call a Professional
Floor sanding is a skill that takes practice. Hiring a professional floor sander is strongly recommended if:
- Your floor is made of native NZ timber (rimu, kauri, matai) — mistakes can be irreversible.
- The floor has significant damage, cupping, or previous repairs that need levelling.
- You're unsure about the existing finish type or whether the floor has been previously sanded (there's a limit to how many times a floor can be sanded before the boards become too thin).
- You want a guaranteed result for a rental property or before sale.
A qualified floor sander will have professional-grade equipment and the experience to read the floor correctly at every stage. You can find local floor sanding professionals on tradedirectory.co.nz.
