My fence has gone grey. Will washing bring the colour back?

Partly, and it depends which grey you have. The Forest Products Laboratory names mildew as the most common biological cause of greying and it washes off. The grey underneath it is weathered wood, and the Forest Products Laboratory is plain that cleaning wood does not add colour — that part stays where it is. Which of the two you have is something we tell you at the estimate, before you commit to anything. If it is the colour you want back rather than the growth gone, you are looking at a finish, not a wash.

There is black staining around every nail and screw. Is that mildew?

Almost certainly not. Blue-black staining that rings a fastener is an iron-tannin reaction: the tannin comes out of the wood — cedar, redwood and oak are full of it — and the iron comes out of the steel fastener. It answers to different chemistry from mildew, and a mildewcide will do nothing to it. A drop of household bleach on the spot is the quick check: mildew fades in a minute or two, and an iron stain just sits there looking at you.

Is bleach used to clean a fence?

On wood, a commercial cleaner or a dilute solution of household bleach with detergent is what the Forest Products Laboratory recommends for mildew, and household bleach is also what its drop test uses. On vinyl, the vinyl siding association lists undiluted chlorine bleach among the products not to use, and individual manufacturers set their own limits on top of that. This is one of those questions with no blanket answer, so tell us what the fence is made of.

We want to stain the fence after. How soon should that happen?

Soon, and this is advice rather than a sale — we clean, we do not paint or stain. Exposure work by the Forest Products Laboratory found wood left bare for sixteen weeks before painting was peeling within about three years, while wood painted after a single week held for roughly thirteen years. Badly weathered wood with loose surface fibres cannot hold paint. That study is about paint and this page does not stretch it to cover a penetrating stain, but the loose fibres are the same fibres either way, so line up whoever is finishing the fence close behind the wash rather than months later.

Fence cleaning

A fence that has gone black and grey can have more than one thing wrong with it, and they are not the same thing. Mildew, plain dirt, the red-brown bleed out of cedar heartwood, and the blue-black ring around every steel fastener all read as one stain from the driveway, and each answers to different chemistry. Then there is the grey itself, which is sometimes not a stain at all. The first thing to find out is which of them you have.

Four different stains that look like one

The Forest Products Laboratory publishes a test you can run yourself before anyone quotes: put a drop of household bleach on the discolouration and watch it. Mildew bleaches out in a minute or two. Discolouration that does not bleach is probably dirt, extractive bleed or iron stain — three different problems with three different answers. Mildew is a fungus, and the useful thing to know about it is that it cannot decay wood. It lives on the extractives in the wood and on the oils in an old finish, and what it leaves behind is a black deposit. It gathers where the air does not move and the surface stays damp: the shaded run, the boards behind a hedge, the bottom of every picket. The blue-black staining that rings each nail and screw is not mildew at all. It is an iron-tannin reaction — cedar, redwood and oak are full of tannin, and a steel fastener supplies the iron. Reddish-brown streaks running down from knots and joints on cedar or redwood are extractive bleed, water-soluble colour washing out of the heartwood; sapwood does not carry those extractives, which is why the same fence streaks on some boards and not others. Oxalic acid handles the iron and the extractive stains and, in the Laboratory's own words, is not effective for removing mildew. Put a mildewcide on an iron stain and you have accomplished nothing. So the test comes first.

Some of the grey washes off and some of it is the wood

This is a sentence worth being exact about. Sunlight breaks down the lignin at the surface of the wood and weathering carries the coloured components away. What is left is grey, and the Forest Products Laboratory is blunt about the consequence: cleaning wood does not add colour. Nothing that can be sprayed on puts it back, and a before-and-after photograph implying otherwise is selling an outcome no wash can deliver. The same chapter, though, names mildew as the most common biological factor in the greying of wood — and mildew does wash off. So the honest answer is that part of the grey on your fence is a living deposit sitting on the surface and will come away, and part of it is the wood itself and will stay. The drop of bleach shows the proportion, which is why it belongs before the quote rather than after. While we are here: the idea that a bare fence weathers into an even silver-grey patina holds in few places. The Laboratory's position is that in most North American climates exterior wood left unfinished develops blotchy mildew growth instead, and that the driftwood look takes years even in the few places it happens at all.

What pressure does to wood, and what a wood cleaner really does

Wood weathers slowly. Left bare, the surface erodes something like a quarter of an inch in a century, because lignin photodegrades and leaves the cellulose fibres loosely attached rather than washed away. A pressure washer does the same erosion mechanically, and it does it unevenly: the soft earlywood inside each growth ring erodes faster than the dense latewood beside it, so the surface comes out ridged. Those ridges do not settle back down, and the Forest Products Laboratory notes they are difficult to repaint. In the Laboratory's own words, high pressure can easily cause deep erosion of wood. The same honesty is owed to what gets sold as restoration. The Laboratory describes commercial cleaners — the sodium-hydroxide ones specifically — that pulp the surface of the wood so that power washing then removes it. The colour looks restored because the grey layer is gone, and it is gone because the wood it belonged to went with it. Sanding, the Laboratory adds, would give the same result. The Laboratory places the gentle brighteners at one end of that range and these sodium-hydroxide cleaners at the other extreme; the pulping belongs to the second group. No trade body or manufacturer we could find publishes a safe pressure for washing wood, so this page does not invent one. What the Forest Products Laboratory recommends for mildew on wood is a commercial cleaner or a dilute solution of household bleach with detergent — and if what you want off the fence is paint rather than growth, that is a different job with its own page.

Not every fence is wood, and not every wood is the same

For vinyl, the vinyl siding trade association's instruction is the stream held straight, at eye level, and never angled upward, because water driven upward gets in behind the panels. The same association notes that some manufacturers do not recommend power washing their product at all, that others set their own pressure and cleaner limits, and that undiluted chlorine bleach is on the list of things not to use on it. So the answer for a vinyl fence starts with whose panel it is. On aluminium and powder-coated panel the finish has to be identified before anything else; tell us what you know about it. The authoritative cleaning standard for architecturally finished aluminium sits behind a paywall we have not bought, so this page makes no technical claims about it on the strength of what everyone says. On wood, the Forest Products Laboratory conditions natural durability on the wood being all heartwood from a decay-resistant species — cedar sapwood is not durable — and notes that naturally durable and preservative-treated woods have quite comparable service lives above ground. Cedar is handsome and it is tannin-rich, which is exactly why it streaks and why it stains around steel. It is not armour, and we are not going to tell you it outlasts treated pine.

Common questions

My fence has gone grey. Will washing bring the colour back?: Partly, and it depends which grey you have. The Forest Products Laboratory names mildew as the most common biological cause of greying and it washes off. The grey underneath it is weathered wood, and the Forest Products Laboratory is plain that cleaning wood does not add colour — that part stays where it is. Which of the two you have is something we tell you at the estimate, before you commit to anything. If it is the colour you want back rather than the growth gone, you are looking at a finish, not a wash.
There is black staining around every nail and screw. Is that mildew?: Almost certainly not. Blue-black staining that rings a fastener is an iron-tannin reaction: the tannin comes out of the wood — cedar, redwood and oak are full of it — and the iron comes out of the steel fastener. It answers to different chemistry from mildew, and a mildewcide will do nothing to it. A drop of household bleach on the spot is the quick check: mildew fades in a minute or two, and an iron stain just sits there looking at you.
Is bleach used to clean a fence?: On wood, a commercial cleaner or a dilute solution of household bleach with detergent is what the Forest Products Laboratory recommends for mildew, and household bleach is also what its drop test uses. On vinyl, the vinyl siding association lists undiluted chlorine bleach among the products not to use, and individual manufacturers set their own limits on top of that. This is one of those questions with no blanket answer, so tell us what the fence is made of.
We want to stain the fence after. How soon should that happen?: Soon, and this is advice rather than a sale — we clean, we do not paint or stain. Exposure work by the Forest Products Laboratory found wood left bare for sixteen weeks before painting was peeling within about three years, while wood painted after a single week held for roughly thirteen years. Badly weathered wood with loose surface fibres cannot hold paint. That study is about paint and this page does not stretch it to cover a penetrating stain, but the loose fibres are the same fibres either way, so line up whoever is finishing the fence close behind the wash rather than months later.

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