How to clean, oil and maintain a Kwila deck in New Zealand

Kwila is a forgiving timber if you stay on top of it. The natural oils in the wood give it built-in resistance to moisture and decay that treated pine simply doesn't have. But those natural oils don't last forever without help, and Tauranga's combination of high UV, summer humidity, and coastal salt air will work through an untreated kwila deck faster than most people expect.

This guide is for kwila deck owners, not people deciding whether to buy kwila. If you're in the research phase, our guide to what kwila decking is covers that ground. This article is for the person standing on their existing deck wondering what to buy, how to apply it, and how often to do it.

Oil or stain: which should you use on a kwila deck?

This is the question we get asked most often, and the answer matters because making the wrong call is difficult to undo.

The case for oil

A penetrating oil soaks into the timber fibres rather than sitting on the surface. It feeds the wood's natural oils, repels moisture from within, and allows the deck to breathe. When it wears off, it does so gradually and evenly, and reapplication is straightforward: clean, let dry, apply another coat.

For kwila specifically, a penetrating oil is almost always the right choice. Kwila has its own natural oil content, and a penetrating oil works with that rather than against it. It also allows the tannins to continue their natural movement out of the timber rather than trapping them, which reduces the risk of discolouration beneath a film-forming product.

The case for stain

A stain sits on or near the surface of the timber and adds colour as well as protection. Pigmented stains can refresh the look of a deck that has already greyed off, restore a more uniform colour to boards that have weathered unevenly, or give a kwila deck a slightly darker or richer tone than the natural colour.

The downside is that stains, particularly film-forming stains, can peel and flake as they age in ways that oils don't. When they need reapplication, you often need to sand back the old product first, which is significantly more work than re-oiling.

The recommendation for kwila

For a new or recently cleaned kwila deck, use a clear or lightly tinted penetrating oil. For a deck that has been neglected and has significant greying or colour variation, a pigmented penetrating stain can help restore a more even appearance before transitioning back to oil.

Avoid film-forming products on kwila. They look good initially but create more maintenance work long term.

Products available in New Zealand

These are products that work well on kwila and are available at Mitre 10, Bunnings, or through trade suppliers in Tauranga:

Cabot's CD50 Kwila Oil is the most widely recommended product for kwila specifically. It is formulated for tannin-rich hardwoods, penetrates well, and is available in clear and tinted versions. Apply two coats within the first seven days of installation on a new deck, and annually or every two years after that.

Deks Olje D1 is a penetrating oil that suits hardwoods well and is popular with builders and tradies. It is water-based, low odour, and dries to a natural finish.

Sikkens Cetol HLS Plus and Cetol Filter 7 Plus are semi-transparent stains with good UV protection. These are more film-forming than a pure penetrating oil, so they suit decks where colour restoration is a priority.

Feast Watson Kwila Decking Oil is a purpose-formulated penetrating oil available at most hardware stores. Good everyday option for maintenance coats.

If you're unsure which product to use for your specific situation, the timber and paint specialists at Resene ColorShops in Tauranga will give you straightforward advice based on your deck's current condition.

How often should you oil a kwila deck?

The honest answer is: it depends on the deck's exposure.

A north-facing kwila deck in full summer sun in Papamoa needs oiling every 12 months. The UV load is simply too high to stretch further than that without the timber starting to grey and dry out at the surface.

A south-facing or well-shaded kwila deck can comfortably go two years between oil coats. The reduced UV and lower temperature variation mean the oil breaks down more slowly.

A simple test: tip a small amount of water onto the deck surface. If it beads and rolls off, the existing oil is still doing its job. If it soaks in within a few seconds, it is time to re-oil.

Do not leave it beyond two years regardless of how protected the deck is. By year three without oil, the surface fibres start to break down and cleaning back to bare timber becomes significantly more work.

Step-by-step: how to clean a kwila deck

Do this before every oil application, and once a year even if you are not re-oiling that season.

What you need: stiff-bristled deck brush, garden hose or low-pressure washer, deck cleaner (30 Seconds, Wet and Forget, or Simple Green Deck and Fence Cleaner all work well), and a bucket.

Step 1: Clear the deck completely. Move all furniture, pot plants, outdoor rugs, and anything else sitting on the surface. Anything left in place will leave a shadow of dirt and mould underneath it.

Step 2: Sweep or blow off loose debris. Get into the gaps between boards where leaf litter and seed pods accumulate. This organic material holds moisture against the timber and accelerates surface mould if left.

Step 3: Apply your deck cleaner according to the manufacturer's instructions. For 30 Seconds, dilute and apply with a sprayer or watering can and leave for 20 to 30 minutes before scrubbing. Do not let it dry on the surface.

Step 4: Scrub with a stiff brush, working along the grain of the timber rather than across it. Pay attention to the edges of boards where mould tends to concentrate.

Step 5: Rinse thoroughly with a hose. Do not use a high-pressure water blaster on kwila. The pressure damages the timber surface fibres and opens up the grain in a way that accelerates future weathering. A low-pressure wash is all that is needed.

Step 6: Allow the deck to dry completely before applying oil. In Tauranga's summer this typically takes one to two dry days. In winter or shaded conditions, allow three to four days. Oiling wet or damp timber traps moisture beneath the oil coat and can cause discolouration.

Step-by-step: how to oil a kwila deck

What you need: Cabot's CD50 Kwila Oil or equivalent penetrating oil, a short-nap roller or quality brush, roller tray, old clothes (kwila oil stains permanently), cardboard or drop cloth for surrounding surfaces.

Step 1: Confirm the deck is completely dry. Run your hand along several boards in different areas. If any boards feel cool or damp, wait another day.

Step 2: Protect surrounding surfaces. Kwila oil will stain concrete, pavers, and painted surfaces. Lay cardboard or drop cloth around the perimeter of the deck before you start.

Step 3: Apply the first coat along the length of each board using a roller or brush. Work in manageable sections of three or four boards at a time. Apply generously enough that the oil soaks in rather than sitting on the surface, but not so heavily that it pools.

Step 4: Allow the first coat to soak in for 20 to 30 minutes, then apply a second coat the same day or within seven days. Two coats applied in close succession penetrate more deeply than two coats applied weeks apart, particularly on a new or recently sanded deck.

Step 5: Wipe off any excess oil that has not soaked in after the final coat. Pooled oil can dry sticky and attract dirt.

Step 6: Keep foot traffic off the deck for at least 24 hours after the final coat. Keep furniture off for 48 hours.

How to deal with a neglected kwila deck that has gone grey

If the deck has been left without oil for several years, the surface will have weathered to grey and the top layer of fibres will have broken down. You cannot oil over this and expect a good result. The grey surface needs to be removed first.

Option 1: Deck brightener or oxalic acid wash. A deck brightener (available at paint stores and some hardware stores) or a diluted oxalic acid solution can restore significant colour to a greyed kwila deck without sanding. Apply according to the manufacturer's instructions, scrub, and rinse. This works well for decks that are grey but not physically damaged.

Option 2: Light sanding. For decks with surface checking (small cracks along the grain), raised fibres, or uneven weathering, a light sand with 80 grit sandpaper before the brightener wash gives better results. Sand along the grain only. This removes the dead surface fibres and opens up fresh timber for the oil to penetrate. Do not sand aggressively or you will remove more timber than necessary.

Option 3: Professional restoration. If the deck has been neglected for a long time or has significant checking, splitting, or structural issues in individual boards, it is worth having a builder assess it before you invest time and money in a restoration. Some boards may need replacing before the rest of the deck is treated.

After restoring the colour, apply two coats of penetrating oil within seven days of the final clean. The freshly opened timber will absorb oil readily and this is the most important oil application the deck will get.

Fixing common kwila deck problems

Tannin stains on concrete or paving

Fresh tannin run-off can be scrubbed off with warm soapy water if caught early. Dried tannin stains on concrete are harder to remove. Scrubbing with diluted oxalic acid solution (available at hardware stores) is the most effective method. On pavers, a deck brightener product applied directly to the stain and scrubbed before rinsing often works well. Milk has been suggested as a traditional remedy for early-stage tannin stains; it works by breaking down the tannins chemically, but it needs to be applied when the stain is still fresh.

To prevent future tannin staining, pre-oil all four sides of each board before installation on any new kwila deck, and ensure the deck drains away from concrete and light-coloured paving where possible.

Surface mould and mildew

A green or black slippery film on the deck surface is surface mould, common on any shaded or poorly ventilated deck in Tauranga's humid summers. It is a slip hazard and should not be left. Clean with 30 Seconds or Wet and Forget applied to the surface and left to work before scrubbing and rinsing. Re-oil after cleaning. Keeping gaps between boards clear of leaf litter significantly reduces how quickly mould returns.

Raised nail or screw heads

Kwila is a hard, dense timber that moves very little with seasonal moisture changes, but it does move slightly. Over time, fixings can work slightly proud of the surface and become a trip hazard. Hammer raised nails flush, or drive screws back down with a drill.

If the same fixing keeps coming proud, the hole may have enlarged and the fixing needs to be replaced with a larger diameter screw into fresh timber nearby.

Checking (small surface cracks along the grain)

Checking is a normal characteristic of hardwood decking and does not indicate a structural problem. It occurs when the surface of the board dries faster than the interior. Consistent oiling reduces checking significantly by keeping the moisture content of the timber more even.

Surface checks in an otherwise sound board do not need filling; they close partially when the timber takes on moisture and oil. Deep or wide checks in individual boards that have been neglected for a long time may warrant board replacement.

Discolouration under pot plants or furniture

Prolonged contact with moisture from pot plant saucers causes dark staining on the deck surface. Move pot plants regularly and use purpose-made pot plant feet that allow air circulation underneath. Furniture legs can leave marks over time; felt pads on furniture legs help. Existing discolouration from pot plants can often be reduced with a deck brightener wash followed by re-oiling.

Kwila maintenance schedule: the simple version

New deck installation: oil all four sides of each board before laying, apply face coat after installation.

Year one: clean in spring, re-oil if the water bead test shows absorption.

Annually: clear gaps between boards of debris, clean with deck wash, inspect fixings and boards for any issues.

Every one to two years: re-oil. Every year for full sun north-facing decks. Every two years for shaded or south-facing decks.

Every five years: assess whether any individual boards need replacing, check subframe bearers and joists for any signs of deterioration at ground level.

Talk to Klinks

If your kwila deck needs more than a clean and re-oil, or if you are not sure whether restoration or replacement is the better call, we are happy to take a look. We build kwila decks across Tauranga, Mount Maunganui, Papamoa, Bethlehem, Omokoroa, and the wider Bay of Plenty, and we know this timber well.

Call Craig: 027 630 2735 Or request a free assessment at klinks.co.nz/contact

Klinks Fences & Decks, 10/45 Kaweroa Drive, Omanawa, Tauranga.

Product recommendations in this article reflect what works well in Tauranga conditions based on our building experience. Always read manufacturer instructions before applying any deck treatment product.