Most weeks over summer I'll walk a cedar home with an owner who's a bit gutted. The cladding looked a million bucks when the build was signed off, and now, two or three years on, it's gone patchy grey on the north face, there are dark streaks running down near the nail heads, and somewhere there's a bit that's started to flake.

The first question is always the same: "What did we do wrong?"

Usually the answer is "nothing, you just left it a season too long." But the better answer, the one that actually helps you make good decisions, is to understand what's happening inside the timber. Because once you get the chemistry of cedar, every weird thing it does outside starts to make sense, and you stop guessing about stains.

So let me take you under the surface. No lab coat required.

Here's the whole thing as one interactive panel. Have a poke at it, tap the bands, drag the sun across the timber, switch the stains and age them, then I'll break it all down below.

What cedar is actually made of

Strip it right back and a cedar board is three structural polymers plus cedar's secret ingredient.

• Cellulose (about 45%) is the strong stuff. Long, crystalline chains of sugar that act like rope, giving the timber its stiffness and strength.
• Hemicellulose (about 22%) packs around the cellulose and binds the cell wall together. It's the most water-loving part, so it drives a lot of the swelling and shrinking as the board wets and dries.
• Lignin (about 28%) is the glue. It's an aromatic polymer that stiffens the cell walls and holds the fibres together. Remember this one, because lignin is the weak link outdoors.
• Extractives and natural oils (the last few percent) are what make cedar cedar. Compounds like the thujaplicins and plicatic acid are naturally fungicidal and put insects off, which is why cedar lasts so well untreated.

Here's the bit most people get wrong, and I'll be honest, I had it slightly wrong for years too. Those famous natural oils protect cedar from rot and bugs, not from the sun. Cedar has no real defence against UV. None. That's the whole reason a coating exists.

Why cedar goes grey (it's the lignin failing)

Silvering isn't dirt, and it isn't rot. It's a chemical breakdown, and it happens in a very specific order:

1. UV smashes the lignin. Lignin's molecular rings soak up ultraviolet light and break apart into water-soluble fragments. It takes the hit so the rest of the wood doesn't.
2. Rain washes it out. The next shower rinses those broken-down lignin bits straight off the surface, leaving the top fibres loose.
3. Bare cellulose is left behind. Cellulose doesn't colour up the same way, so a surface that's lost its lignin reads as that classic silver-grey.
4. Then the trouble starts. A roughened, lignin-poor surface drinks water, and that's when warping, checking and splitting kick off.

That's the key thing about greying up here. On its own it's mostly cosmetic, the timber underneath is usually still sound. But it's the early warning light. Up at 300 to 400 metres with our hard nor'west UV, cedar moves through these stages fast, especially on north and west faces. Catch it at stage one or two and it's a clean-and-recoat. Leave it to stage four and you're into stripping, or worse, replacing split boards.

Why cedar goes black (there are two different culprits)

This one trips people up, because "black" has two completely different causes and they need different fixes.

Culprit one: the iron reaction. Cedar is acidic and loaded with those tannin-like extractives. When water carries them into contact with iron from a non-stainless nail or screw, they bond into a dark iron-tannate complex. It's literally the same chemistry old iron-gall ink was made from. The tell is simple: if the black runs in streaks directly below the fixings, that's the iron reaction, and it means someone used the wrong fasteners. Cedar needs stainless steel, full stop. (And never take steel wool to cedar, the stray filings rust and freckle the whole wall.)

Culprit two: the weathering fungus. A lot of the general grey-black film you see across a tired cedar face isn't the iron reaction at all. It's a fungus that colonises the surface once the protective extractives have been weathered out of it. In other words, it moves in after the cedar has silvered and lost its natural defences. That black tells you the surface was left too long without a coat. It cleans and treats off, then needs recoating to stop it coming back.

So on a site walk I'm always asking: is the black lined up under the fixings (iron, wrong nails), or is it a general film across a sun-beaten face (fungus, overdue for a coat)? Different cause, different conversation.

Where stains actually live: the whole game in one question

Here's the thing that took me years to properly internalise, and it makes choosing a product easy once it clicks.

The only real difference between the stain families is where the coating physically sits relative to the wood surface. That single fact decides whether a finish fades quietly or fails dramatically.

Penetrating oil stain (the workhorse)

Think Resene Woodsman. The oil carries pigment down into the top few millimetres of the timber. There's no film sitting on top, the colour lives inside the wood and lets the grain show through. Because nothing sits on the surface, there's nothing to peel. It just fades as the pigment wears, and you clean it and recoat. Waterborne is easier to apply and clean up; the solventborne version penetrates deeper and suits more weathered, thirsty cedar.

Film-forming stain (avoid it on cladding here)

This is heavy on binder, so it cures into a continuous skin on top of the timber, almost like a thin paint. Looks fantastic on day one. The problem is the wood never stops moving underneath it, the rigid film can't follow, water sneaks under, and our UV and big day-night temperature swings embrittle it. The bond fails, and now it cracks and peels. Once a film coating goes, you can't just recoat, you're stripping back to bare timber. In Wanaka especially, this is the product that turns a tidy maintenance job into an expensive nightmare. If you've already got peeling on your hands, I walk through the fix in The Kiwi Guide to Sorting Out Peeling or Bubbling Paint.

Migrating oil (the premium option)

Dryden WoodOil and Wood-X are the ones here. These are penetrating oils engineered to stay mobile deep within the board, re-distributing through the timber rather than curing hard. They repel water from the inside, which keeps the cedar dimensionally stable, so less cupping and splitting. No surface film means no peeling, ever. They cost a bit more and the recoat windows differ by brand, but on a high-value cedar home they're hard to beat.

Brad's take: If a product forms a hard film on the surface of your cedar, it can fail catastrophically. If it soaks in, the worst it can do is fade and ask for a recoat. That's the whole reason penetrating and migrating products own this market.

Pigment is sunscreen (and the catch nobody mentions)

This is where I see the most confused decisions, so stick with me.

It's the pigment, not the oil, that actually blocks UV from reaching the lignin. So the more colour you carry, the more UV protection you get:

• Clear or natural shows off the most grain and dies the youngest. Almost no pigment, so UV drives straight down to the lignin. Gorgeous, and the fastest to silver.
• Semi-transparent is the cedar sweet spot. Enough pigment to block most of the UV while the grain still reads through. This is why these dominate cladding.
• Opaque holds colour the longest because there's the most pigment to lose, but you've covered up the timber's natural character.

Now the catch, and this is the question a client asked me last week that's worth answering properly. If pigment blocks UV, surely a deep black stain is the most protective of all?

On the UV side, yes. But black absorbs heat as well as blocking UV. A dark cedar wall in full Central Otago sun can sit 20 to 30 degrees hotter than a pale one, comfortably 60 to 70°C on the surface. That heat is a separate enemy. It drives bigger expansion and contraction, more moisture cycling in and out of the board, and it cooks the coating's binder faster, even while the UV is being blocked nicely.

So a black stain is the best at stopping UV and the worst at managing heat. That's the contradiction people feel and can't put their finger on.

The fix is clever: CoolColour pigments (Resene offers them across the Woodsman range) reflect the infrared, so the board stays cooler while keeping the dark colour you wanted. On a sun-hammered elevation in a dark colour, that's the right call, not a regular black.

What this means for your cedar, practically

Pull all that chemistry together and the maintenance plan basically writes itself:

• Wash it yearly. A gentle soft wash keeps grime and fungus off and resets the clock. Never water blast cedar, it's a soft timber and you'll fur the grain.
• Recoat before it silvers, not after. Colour fade is your signal that the pigment sunscreen is wearing through and UV is starting to reach the lignin. North and west faces here usually want it every 2 to 3 years; sheltered south and east faces can stretch to 3 to 4.
• Match the product to the timber and the aspect. Penetrating or migrating for cladding, a CoolColour for dark colours on hot faces, and walk away from film stains.
• Sort the cause of any black before you coat over it. Iron streaks mean a fixing problem; a fungal film means it was overdue. Coating over either without treating it just buries the problem.
• Use stainless fixings. Cheaper nails are a false economy you'll read on the wall for the life of the cladding.

The honest bottom line

Cedar isn't high-maintenance, it's just un-forgiving of being ignored. Understand that UV eats the lignin, that black has two different causes, and that the right stain is the one that soaks in rather than skins over, and you'll get decades out of your cladding looking the way it should.

If you're staring at a faded or streaky cedar home around Wanaka or Queenstown and you're not sure which stage it's at, that's exactly the call I love getting. I can usually tell you on the spot whether you're looking at a straightforward recoat or something that needs more work, and roughly what it'll cost. Get in touch and I'll come take a look.

Look after the chemistry, and the cladding looks after itself.

— Brad