When do balconies and wooden decks need sanding?
Most wooden decks and balconies can be kept in good condition with a thorough cleaning and a coat of oil every spring. However, there comes a point where cleaning and oiling are no longer enough. The wood has turned grey, rough, and splintered, and applying new oil on top of a degraded layer won't help. At this stage, the wood needs sanding. In this guide, you'll learn the specific signs that sanding is necessary, what sanding actually does, how the process works, and when it makes sense to leave the task to professionals.
Cleaning and oiling or sanding: what's the difference?
Cleaning and oiling maintain a surface that is fundamentally sound. You remove dirt, algae, and old, loose oil, and apply new protection to wood that is still smooth and intact. This is the routine most decks need to go through once a year. If you want the full procedure, you can read our guide to oiling your deck.
Sanding removes the damaged surface layer of the wood itself. It strips away years of wear and weathering to reveal fresh wood ready for treatment. This is the next step when the wood surface, as well as its protective finish, is worn.
Signs that sanding is necessary
You can check for these signs yourself:
- The grey colour runs deep into the surface. A grey surface means UV light and weather have broken down the outer wood fibres. Cleaning may remove some of the grey, but a deeper weathered layer needs sanding to reveal fresh wood.
- Splinters and rough surface. Splinters and raised fibres make a deck uncomfortable to walk on barefoot. They are a clear sign that the surface needs smoothing.
- Old oil or varnish is peeling. When an old treatment flakes or peels in patches, you can't just oil over it. The loose layer must be sanded off so that the new oil can penetrate clean wood.
- Raised fibres after cleaning. If the wood feels rough and fuzzy after you've cleaned and let it dry, it's often a sign that sanding is necessary before oiling.
- Uneven surface. Sanding removes marks from old oil and paint and smooths a surface that looks and feels uneven.
Check how water behaves on the wood. If it still forms beads and the surface is sound, cleaning and oiling may be enough. If it soaks in and leaves dark patches, the wood is unprotected and more likely to become grey and rough.
What does sanding do to the wood?
Sanding removes the grey, weathered layer and reveals fresh wood. It also prepares the surface in three useful ways:
- The wood is ready to absorb treatment. Oil and wood protection products struggle to penetrate an uneven, weathered surface. Sanding exposes fresh wood that can absorb the finish properly.
- The surface becomes smooth and safe. Removing splinters and raised fibres makes the deck comfortable to walk on and slip-resistant when dry.
- The appearance becomes more even. Discolouration and unevenness are sanded away so the surface looks consistent before finishing.
Good preparation helps the new finish last. Oil applied to a weathered layer cannot penetrate properly and wears away quickly.
Grit and sandpaper: the step-by-step process
Sandpaper is numbered according to grit size. The lower the number, the coarser the paper. Sanding wood always proceeds from coarse to fine in a fixed sequence, and a basic rule is not to skip a grit. Each step removes the scratches left by the previous one until the surface is smooth and ready.
| Step | Typical grit | What it does |
|---|---|---|
| Coarse sanding | 40-60 | Removes the old finish, grey surface layer and larger uneven areas. |
| Medium sanding | 80-100 | Removes light discolouration and smooths the surface. |
| Fine sanding | 100-120 | Creates a smooth, uniform surface ready for oil or wood preservative. |
A very grey, worn deck needs a coarse grit to reach fresh wood, followed by progressively finer paper. Wood in better condition may allow a finer starting grit. The right choice depends on the species and how weathered the surface is.
Sanding step by step
The process itself is similar from deck to deck, whether it's a balcony or a large garden deck:
- Clean and dry. Scrub away dirt, algae and old oil, then let the deck dry completely before sanding. Allow at least 6 to 12 hours after cleaning, and make sure the wood is no longer damp.
- Coarse sand. Remove the grey, weathered layer and old finish with a coarse grit.
- Sand up through the grits. Medium and fine sanding, so the surface gradually becomes smoother.
- Sweep and vacuum. All sanding dust is removed so it doesn't mix with the oil.
- Oil. Protect the freshly sanded wood with oil. We explain how in our guide to oiling your deck.
Spring or early summer is a good time, when the wood is dry. You need a dry spell for both sanding and oiling so the treatment can cure properly.
Safety: splinters and slip resistance
Safety is a good reason to sand a worn deck. Rough wood can cause splinters, and a dirty, weathered surface can become slippery when wet. Cleaning, sanding and a fresh oil treatment make the deck more comfortable to walk on and improve its slip resistance. On a balcony several floors above ground, keeping the surface in good condition is particularly important.
Cleaning, sanding or replacement: a decision guide
Whether your deck needs cleaning, sanding or repairs depends on the extent of the weathering and damage. Use this overview as a guide:
| Condition | Signs | Solution |
|---|---|---|
| Sound surface | Water beads, wood is smooth, only dirt and algae on top | Cleaning and oiling |
| Worn surface | Grey wood, splinters, raised fibres, peeling oil or lacquer, and an uneven surface | Sanding and then oiling |
| Deteriorated wood | Rot, soft boards, deep cracks, boards breaking or giving way | Replacement of affected boards |
Most decks that seem hopeless can be saved with sanding. Typically, only individual rotten or broken boards actually need to be replaced, and the rest can be sanded and oiled to look like new. If you are in doubt about what your deck needs, you can get a no-obligation quote for balcony and deck sanding, and we will assess the wood and give you a fixed price before we start.
Finishing: oiling is part of the job
Sanding and oiling work together. Sanding exposes bare wood that needs protection from moisture and weather, so oiling should follow as part of the job. Read more about the finishing process in our guide to oiling your deck and about ongoing care in our guide to maintaining your balcony.
The interval between later coats depends on the wood species. Pressure-treated wood and larch usually need oil once a year, while hardwoods such as ipé can often go longer. To have the maintenance taken care of for you, sign up for an oiling plan, where we come and oil at the right time.
Balconies in shared buildings: a consistent appearance
Professional sanding can give balconies across a housing association or larger building a consistent appearance. Using the same method and finish creates a coordinated facade instead of a patchwork of different colours and wear. We work throughout Denmark and have strong references in Aarhus, including Isbjerget. We can handle the whole association's project for a fixed price. Read more on our balcony sanding page.
When should you hire professionals?
A small balcony in reasonable condition may be manageable as a DIY project. For larger areas, damaged boards or awkward access, professional assessment helps establish the right approach and a fixed price. Balcony sanding and oiling are not listed as eligible services in the Danish Tax Agency's 2026 deduction guidance. See our tax deduction information and read more about balcony sanding.
Frequently asked questions
When should a wooden deck be sanded instead of just cleaned?
Sanding is needed when grey weathering runs deep into the surface, the wood is rough or splintered, or an old finish is peeling. Cleaning and oiling alone may no longer be enough. If the surface is sound and water still forms beads, routine cleaning and oiling are usually sufficient.
What grit size is used to sand a wooden deck?
You work from coarse to fine in a fixed sequence. Typically, you start with grit 40 to 60 to remove the grey layer and irregularities, medium sanding with grit 80 to 100, and fine sanding with grit 100 to 120. You never skip a grit.
Can a grey and worn deck be saved, or does it need to be replaced?
Most grey and worn decks can be saved with sanding. Typically, only individual rotten or broken boards actually need to be replaced. The rest can be sanded down to fresh wood and oiled to look new again.
Should a deck be oiled after sanding?
Yes. Sanding exposes bare wood that needs protection from moisture and weather. Apply oil shortly afterwards as part of the finishing process.
How often should a wooden deck be oiled after it has been sanded?
It depends on the type of wood and how exposed the deck is. Pressure-treated wood and larch will typically need it once a year, often in the spring, while hardwood can manage with longer intervals. The more sun and rain, the shorter the interval.