Removing wooden floor stains: seven common types
Most stains on a wooden floor can be removed without sanding the entire floor. Black stains are caused by tannic acid and are treated with oxalic acid, light water stains often dry out and are re-oiled, and grease can be removed with floor soap in warm water. If the discolouration has penetrated the wear layer, the floor must be sanded.
- Black rings and spots are almost always tannic acid bleed from water plus metal, typically on oak. Junckers recommends approx. 100 g oxalic acid dissolved in 1 litre of warm water (junckers.dk, accessed 2026).
- Light, whitish water stains are on the surface finish. Dark stains are in the wood itself and require chemicals or sanding.
- Oiled floors can be spot-repaired. Lacquered floors cannot, because the lacquer film must be broken in an entire strip or the entire room, otherwise the transition will be visible.
- Standard floor sanding with a standard finish costs 175 kr per m2 including VAT from 36 m2. Smaller jobs use fixed area-based prices.
- Øens HåndværkerCentral estimates that a solid wooden floor can withstand 4-6 full sandings in its lifetime, so spot treatment first and sanding afterwards.
- 175 kr/m2Including VAT and a standard finish, from 36 m2
- 24 hoursReply with fixed price
- Workmanship guaranteeOn all work performed
Table of contents for this guide
- What kind of stain do you have on your floor?
- Why do stains on wooden floors turn black?
- How do you remove water stains from a wooden floor?
- What do you do about grease, red wine, and urine?
- When can the stain not be washed away?
- How much does it cost to sand away the stains?
- Can you sand the stain away yourself?
- How to avoid new stains
What kind of stain do you have on your floor?
Which method works on a stain on a wooden floor depends on whether the stain is in the surface finish or in the wood itself. Light streaks and grease are most often in the lacquer or oil film and can be washed or re-oiled away. Black stains, urine, and deep water stains have penetrated the wood fibres and require either chemical treatment or sanding down to clean wood.
| Stain type | Typical cause | What you can try yourself | When the floor needs sanding |
|---|---|---|---|
| Black spots and rings | Water plus metal on tannin-containing wood, e.g., flowerpot, damp doormat, nails or screws | Oxalic acid, approx. 100 g per litre of warm water, brushed on and rinsed off | If the stain does not lighten after 2-3 treatments, or if the area afterwards appears lighter than the rest |
| Light, whitish water stain | Moisture trapped in the lacquer film or in the oil saturation | Drying out, floor soap in warm water, then re-oiling the area | If the stain is under the lacquer and extends over several boards |
| Dark water stain | Water penetrated through joints, e.g., dishwasher or waterbed | Dry thoroughly first, then oxalic acid | Almost always with stains larger than an A4 page |
| Pet urine | Ammonia reacts with the wood's tannic acid and penetrates deeply | Enzyme-based cleaner, then thorough drying | Often, because the smell is in the wood and not just on the surface |
| Grease and oil stains | Cooking oil, candle wax, shoes, and bicycle tyres | Floor soap in warm water, on oiled floor followed by new oil | If the grease has penetrated the wear layer and appears dark after drying |
| Red wine, coffee, and juice | Dye in the wood's pores, worst on lye-treated and lightly oiled wood | Dab up immediately, wash with floor soap, re-oil lightly afterwards | If the colour is still visible when the floor is dry |
| Black streaks and marks | Sand and grit ground into the lacquer under shoes | Cannot be washed away, because it is wear and not dirt | Yes, the lacquer is worn through and needs to be renewed |
Why do stains on wooden floors turn black?
Black stains on wooden floors are caused by tannic acid bleed. The wood's natural tannic acid reacts chemically with moisture and metal, forming a dark discolouration. This phenomenon is most pronounced on oak, which has a high tannin content. This is according to Junckers' guide on stains in wooden floors (accessed 2026), which points to flowerpots, damp doormats, and metal in contact with water as the typical culprits.
The treatment is oxalic acid, which neutralizes the tannic acid. Junckers suggests approximately 100 g of oxalic acid dissolved in 1 litre of warm water, applied with a brush and rinsed off when the stain has lightened. Oxalic acid is hazardous and corrosive to the eyes, so gloves, tightly fitting protective eyewear, and ventilation are not optional.
A practical drawback is the difference in shade. A treated patch may look lighter than the aged floor around it. On a floor that is 15 to 20 years old, a dark mark can therefore be replaced by a lighter patch. If there are also cracks and gaps between the boards, water settles in the joints, and the stain reappears.
How do you remove water stains from a wooden floor?
Water stains on wooden floors are removed according to their colour. If the stain is whitish, the moisture is in the surface treatment, and it usually disappears by itself within a week when the room is dry. If the stain is dark, the water has penetrated the wood, and the floor must dry completely before you do anything.
Timing is everything. According to Gulvbranchen's Floor Facts on wooden floors and moisture (Danish Industry, last updated 2009), wooden floors are delivered from the factory with an average moisture content of approx. 8%, while the moisture content in late summer can rise to approx. 13% when the relative humidity reaches 65%. If you treat a dark stain while the wood is still wet, you lock the moisture under the new oil or lacquer, and the stain becomes permanent.
On oiled floors, you can lightly sand the stain with fine sandpaper in the direction of the wood grain and re-oil the area. The oil should dry for approximately 24 hours before light use. This method does not work on lacquered floors: you break the lacquer film, and the transition will be visible. At least an entire strip of lacquer must be sanded off, or often the entire room. The difference between the two surfaces is described in our guide on lacquering or oiling a wooden floor.
What do you do about grease, red wine, and urine?
Grease, red wine, and urine should be removed quickly because they penetrate the pores. Always dab with a dry cloth instead of rubbing, otherwise you will push the dye further into the wood. Then wash with floor soap in warm water and let the floor dry completely before assessing the result.
An oiled floor, in Øens HåndværkerCentral's experience, needs re-treatment approximately every 1-2 years to remain saturated. On a floor with intact oil, you can wipe up spills before they penetrate. If the oil is worn away in the kitchen or in front of the dining table, red wine and grease will go directly into the wood. See how to maintain saturation in the guide on maintenance of oiled wooden floors, and avoid common mistakes in washing and cleaning wooden floors.
Urine from dogs and cats is the most difficult. The ammonia reacts with the wood's tannic acid, the discolouration penetrates deeply, and the smell is in the wood rather than on the surface. Enzyme cleaners help with fresh accidents, but for old stains, sanding down to clean wood is typically the only solution that also removes the smell.
When can the stain not be washed away?
The stain cannot be washed away when it is in the wood and not on the surface. Rule of thumb: pour a drop of water on the stain. If it remains as a bead, the surface is intact, and the stain is superficial. If the drop is absorbed and darkens the wood, the protection is gone, and the discolouration is in the fibres.
A lacquered floor, in Øens HåndværkerCentral's experience, typically lasts about 10 years before it needs re-sanding. If the lacquer is worn through to the wood in walkways and in front of the kitchen counter, spot treatment makes no sense because there is nothing left to protect. Then the choice is between light sanding and full sanding, and the difference is described in the guide on light sanding, full sanding, or planing.
Sanding may be an option if enough usable wood remains. We estimate that solid wooden floors typically tolerate 4 to 6 full sandings over their lifetime; engineered flooring allows fewer, depending on wear-layer thickness. If you are unsure about your floor, see our guide to how many times a wooden floor can be sanded provides a concrete answer.
How much does it cost to sand away the stains?
Standard floor sanding costs 175 kr per m2 including VAT from 36 m2, including a standard finish. We assess stains and their depth before confirming what sanding can achieve. Any additional treatment or repairs are agreed in the quotation; deep staining does not always disappear completely.
For floors in Aarhus Ø, including Nicolinehus and Kanalhusene, the right treatment depends on whether marks are in the finish or in the wood itself. At the current standard rate, illustrative sanding-and-finish totals are 7.875 kr for 45 m2, 11.375 kr for 65 m2 and 14.000 kr for 80 m2, including VAT. These are guide calculations, not prices assigned to past stain-removal jobs. A light refresh and deeper repairs are assessed separately. See our floor sanding price guide.
Include drying time in your planning: oil approx. 24 hours before the floor can be used lightly, and lacquer 1-3 days depending on the number of layers. All work is covered by our workmanship guarantee.
Can you sand the stain away yourself?
You can certainly spot treat yourself with floor soap, oil and fine sandpaper, but machine sanding an entire floor is a different matter. The Danish Working Environment Authority's working environment guide for wood and furniture (accessed 2026) states that dust from both hardwood and softwood is carcinogenic and can cause nasal cavity cancer at high dust concentrations over a longer period. A rented belt sander without effective extraction fills the home with precisely that dust.
The technical risk is just as concrete. A belt sander that remains stationary creates a depression that cannot be sanded out again, and which only becomes visible when the lacquer is applied. Therefore, self-sanding often results in a floor that is both stained and wavy. These considerations are outlined in the guide on floor sanding: DIY or professional.
If you need a professional to look at it, we offer professional floor sanding and floor treatment nationwide, based in Aarhus Ø. Call us at 20 98 88 11 or write to us, and you'll receive a response within 24 hours and a fixed price before work begins.
How to avoid new stains
You can avoid new stains by keeping moisture away and the surface saturated. Wooden floors move with humidity: Floor facts from Gulvbranchen (Danish Industry, last updated 2009) estimate approx. 0.22%, equivalent to 2.2 mm per metre, for each percentage point the wood's moisture content changes, and describes late summer as the most humid period, where humidity can reach 65%. A stable indoor climate results in fewer open joints where water can settle.
- Place saucers and felt pads under all potted plants, and move them when watering.
- Avoid rubber backing on runners and doormats, as moisture collects underneath them.
- Wipe up spills immediately, especially on oiled and lye-treated floors.
- Use a well-wrung cloth, never a wet mop, and avoid all-purpose cleaner on wooden floors.
- Re-oil exposed areas before the oil is gone, for example, through an oil maintenance plan.
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Frequently asked questions
Can you remove black stains from wooden floors without sanding?
Yes, in many cases. Black stains are caused by tannic acid bleed and are treated with oxalic acid, approx. 100 g per litre of warm water according to Junckers' guide on stains in wooden floors (accessed 2026). If the stain does not lighten after 2-3 treatments, or if the area appears lighter than the aged floor around it, sanding gives the best result.
Why do water stains on wooden floors turn white?
White water stains occur when moisture is trapped in the lacquer film or in the oil saturation and refracts light differently. The wood itself is undamaged. The stain often disappears by itself within a week in a dry room. Dark stains, on the other hand, are water deep in the wood fibres and require chemicals or sanding.
Can a single board be spot sanded?
On oiled and lye-treated floors, yes, because oil can blend into the surrounding area and the transition is blurred. On lacquered floors, no: the lacquer film is continuous, and spot sanding leaves a visible edge. At least an entire strip must be sanded, and often the entire room.
How much does it cost to have stains sanded off a wooden floor?
Standard floor sanding with a standard finish costs 175 kr per m2 including VAT from 36 m2. Illustrative totals are 7.875 kr for 45 m2, 11.375 kr for 65 m2 and 14.000 kr for 80 m2. The final quotation confirms the stain assessment and any additional work; smaller areas use fixed prices.
How long should the floor dry after treatment?
Oil should dry for approx. 24 hours before the floor can be used lightly. Lacquer requires 1-3 days depending on the number of layers. Wait to put furniture and carpets back until after the full drying time, otherwise you risk new marks and stains on the fresh surface.
Frequently asked questions
Can black stains be removed from wooden floors without sanding?
Why do water stains on wooden floors turn white?
Can a single board be spot sanded?
How much does it cost to have stains sanded off a wooden floor?
How long should the floor dry after treatment?
- Junckers: How to remove stains from wooden floors, junckers.dk (accessed 2026)
- Flooring Industry Association, Danish Industry: Flooring Facts, "General conditions for wooden floors" (last updated 01.05.2009)
- Danish Working Environment Authority: Working Environment Guide for Wood and Furniture, at.dk (accessed 2026)
Øens HåndværkerCentral
We sand and surface-treat floors, stairs, balconies, wooden decks and worktops - based in Aarhus Ø and with jobs nationwide. You get a fixed price before work begins and a workmanship guarantee on the work performed.
Do you live in Aarhus or Copenhagen? See our local pages on floor sanding in Aarhus and floor sanding in Copenhagen with price examples and references from your area.