Merbau is the default hardwood decking across Sydney, and it has one characteristic that sets it apart from every other timber we work on: it leaks.
The dark red-brown staining that appears on concrete, pavers and render below a new Merbau deck is tannin, and it is the single most common complaint about the timber. It is also entirely predictable.
Why it bleeds
Merbau is high in water-soluble tannins and extractives. In the first year or two after installation, every time the deck gets wet those compounds leach out and run with the water.
Wherever that runoff lands and evaporates, the tannin is left behind as a rust-coloured stain. Under the deck edge, down the face of a retaining wall, across a path, onto the pool surround.
Pre-oiling boards on all four faces before installation reduces it substantially. Almost nobody does this, because it doubles the labour, which is why the problem is so widespread.
It also reduces over time as the available extractives deplete. A ten-year-old Merbau deck barely bleeds at all. A new one bleeds heavily for the first couple of wet seasons.
Removing tannin stains
The important thing is that tannin is not rust, despite looking identical, and the products differ.
On concrete and most masonry, an oxalic acid based cleaner is the usual answer. It is sold as deck brightener and as rust and tannin remover. Apply, let it dwell, agitate, rinse thoroughly.
Oxalic acid is mild as acids go but it is still an acid, which rules it out on some surfaces. On sandstone, limestone and travertine — all calcium carbonate — acid etches the stone, and the etch is permanent. Use a dedicated pH-neutral tannin remover instead and accept a slower result. Our piece on rust stains on concrete and stone covers the same distinction.
Do not reach for chlorine. Hypochlorite does very little to tannin and will damage the surrounding timber and plants for no benefit.
And treat it sooner rather than later. Fresh tannin staining comes off readily. Two years of accumulated bleed on porous concrete is a much harder job.
Cleaning the deck itself
Merbau is dense, which helps, but the surface still greys under UV and holds mould in the shade.
Low pressure only. Even on a hardwood, pressure above about 1200 psi erodes the softer spring growth between the harder grain lines, leaving a furrowed, fuzzy surface that then holds dirt worse than before. We cover this in timber decks: when to clean, when to sand.
Use a purpose-made deck cleaner. An oxygenated cleaner for general greying and grime; a dedicated mould treatment where there is growth. Both work better than pressure.
Work along the grain, board by board, with a deck brush.
Rinse thoroughly, including under the boards where runoff collects.
If the deck has gone silver-grey and you want the colour back, an oxalic acid brightener after cleaning restores a lot of it. That is also the step that drives the tannin onto whatever is below, so wet down and protect the surrounding surfaces first.
Fixings and black stains
Black marks radiating around screw heads are a chemical reaction between the tannin in the timber and iron in the fixing. It means the screws are not stainless, or the coating has failed.
Oxalic acid will lift the stain. It will come back unless the fixings are replaced, because the reaction is ongoing.
On a coastal deck this is near-inevitable with anything less than 316 stainless, and it is worth knowing before the deck goes down rather than after. See what coastal Sydney does to a home’s exterior.
Before re-oiling
Oil applied over a dirty or damp deck is the commonest reason a finish fails in a season, and we have written it up separately in Merbau deck oil: when to re-oil and what has to happen first.
The short version: clean, brighten if needed, rinse properly, then wait for the timber to be genuinely dry through — not surface-dry. In Sydney that is typically two to three clear days, longer after rain or in winter.
Protecting what is underneath
If the deck is new, assume it will bleed and plan for it.
Sealing the concrete or pavers below gives you a surface the tannin sits on rather than soaks into, which turns a permanent stain into a wash-off. That is worth doing before the first wet season, not after — see choosing a concrete sealer.
Hosing the area down after heavy rain in the first year, before the runoff dries, also makes a real difference and costs nothing.
For an established deck with staining below it, see deck cleaning — the deck and the surfaces under it are usually one job.
