How to Clean a Teak Deck Without Acid — and Stop Black Mould Coming Back
Every teak deck in the tropics comes out of the wet season the same way: grey turning green in the shade, black spots spreading along the seams, and a slippery film where the water sits. The traditional answer is a bucket, a stiff brush and a two-part cleaner. It works for a week — and every time it is done, a little more of the deck goes over the side with the rinse water.
There is a gentler way, and it is the one many Northern European yards use: kill the growth in the grain instead of scrubbing the surface, and let the teak clean itself. This guide is the whole method, written for boats in Thailand and the rest of tropical Asia.
I have worked on yachts all my life, and Boracol is the only thing I put on a teak deck. No harsh acid eating into the grain, no scrubbing the deck thinner every season — you put it on, it kills the black mould where it lives, and the teak comes back clean and silver by itself. That is why we brought it to Thailand.
Matt Sheldrake, founder, Drake Marine
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What is actually growing on your teak
Three different things share a dirty deck, and they behave differently:
- Algae — the green film in shaded, damp areas. It sits on the surface and makes the deck slippery.
- Black-spot mould — the black speckles that follow the grain and the seams. It is a fungus, and its roots grow down into the timber, which is why scrubbing lightens it but never removes it.
- Lichen — the grey-green crusts on decks and handrails that are left alone for a season or two.
All three feed on moisture, warmth and shade — which a Thai wet season supplies in quantity for five or six months a year.
Why acid and hard scrubbing wear a deck out
Teak is made of hard growth rings with softer timber between them. A two-part acid or caustic cleaner brightens the deck by lifting the grey surface, and a stiff brush or pressure washer then removes it — along with some of that soft timber. Do it every season and the deck becomes ridged, the caulking stands proud, and the planks get measurably thinner. On a laid deck there is only so much teak above the fastenings.
None of that removes black-spot mould at the root, either. The deck looks bright for a few weeks; then the spots come back from inside the grain.
The no-acid method, step by step
- Rinse. Hose the deck down to shift loose dirt and salt. If you need a brush, use a soft one and work across the grain, never along it. No pressure washer.
- Let it dry to damp. The timber should be damp, not wet, so it can absorb the treatment. Pick a dry spell — rain within 24 hours can wash the treatment off before it has soaked in.
- Apply until saturated. Brush, roller or a coarse low-pressure sprayer, until the deck is visibly wet but not running. Keep run-off away from varnish, paintwork and the water.
- Second coat where it is bad. On heavy black spot or very dry, thirsty timber, let the first coat dry back to damp and apply again until the timber will take no more.
- Leave it alone. The growth dies over the following days and weathers away; the deck returns to an even silver. Do not rinse it off. If you want to oil or seal, wait until it has fully dried.
We use Boracol 5Rh for this: a clear, ready-to-use mix of a quaternary biocide, which kills the algae, mould and lichen, and disodium octaborate, which stays in the timber as a preservative against rot and woodworm. It is practically odourless, low-VOC, non-flammable and does not stain.
The tropical calendar: twice a year
| When | Why |
|---|---|
| October–November, straight after the rains | Kill everything the wet season has seeded, and go into the high season with a clean, non-slip deck. |
| April–May, before the rains return | Put protection into the timber before five months of damp, so the growth never gets established. |
Boats laid up on the hard or under covers through the wet season benefit most from the April–May treatment: shade plus trapped humidity is exactly what mould likes.
How much you need
One litre treats 2–4 m² per application, depending on how porous the timber is; plan on 3 m² per litre. A 5 L pack therefore treats about 15 m² once. A 40 ft yacht with 20–25 m² of teak needs about two packs per treatment, four for the year. Our Boracol coverage calculator takes each deck section and the season plan and puts the right number of packs in your cart.
Not just the deck
The same treatment works on the other things that go black on a boat in the tropics: canvas covers and biminis, cushions, GRP and non-slip, timber trim, and the jetty or pontoon you step off. Treat them in the same session.
What owners and designers say
Just wanted to let you know that Boracol is the real deal. Really no trace of black mould at all, even after about eight months. Really pleased — the first treatment I have found that works.
Dennis Stanfill, Director, HB Design
Handling
Boracol 5Rh is a biocide. Wear gloves and eye protection, keep it off skin, keep children and pets off the deck until it is dry, and do not let run-off go into the water. In Thailand it is a regulated hazardous substance; we supply it in the original pack with the Thai label and the safety data sheet.
Foire aux questions
How do you clean a teak deck without acid?
Rinse off the dirt and salt with water and a soft brush worked across the grain, let the deck dry until it is just damp, then apply a borate biocide such as Boracol 5Rh until the timber is saturated and leave it. The treatment kills the algae and mould in the grain, and the dead growth weathers away over the following days and weeks — no acid, no neutraliser and no hard scrubbing.
What kills black mould on a teak deck?
Black-spot mould grows into the grain, so it needs a treatment that soaks in and kills it rather than a wash that cleans the surface. Boracol 5Rh combines a quaternary biocide that kills the mould with a boron preservative that stays in the timber. The black spots fade as the dead mould sheds.
Should I pressure-wash a teak deck?
No. A pressure washer tears out the soft grain between the hard growth rings, leaves the deck ridged and drives water into the seams. Rinse with a hose and use a soft brush across the grain instead.
How often should a teak deck be treated in Thailand?
Twice a year: straight after the rains in October–November, so the deck is clean for the high season, and again in April–May before the rains return, so the growth never gets established.
How much Boracol do I need for my deck?
One litre treats 2–4 m² per application; plan on 3 m² per litre. A 40 ft yacht with 20–25 m² of teak needs about two 5 L packs per treatment. The Boracol coverage calculator works it out from your deck sections.
Will Boracol change the colour of the teak?
No. It is clear and non-staining. The teak returns to an even natural silver as the dead growth sheds; if you want the golden colour back, oil or seal once the treatment has dried.
Get your deck ready for the season
Genuine ACS Boracol 5Rh, in stock in Trang and delivered nationwide in 1–3 days. Tell us your deck size and we will tell you how many litres — or use the calculator.
Boracol 5Rh — 5 LHow much do I need?LINE @drakeco
