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A wood fence wants 500 to 1500 PSI through a 25-degree green tip, held 12 to 18 inches back, worked with the grain from the top down. Budget half a day for a typical suburban run, a bottle of oxygenated deck-and-fence cleaner, and two to three dry days afterward before staining. The damage to avoid is raised grain — once the soft early wood is torn out from between the harder grain lines, the fuzzy surface is permanent, and it takes stain badly forever after.
Almost everything that goes wrong here comes from treating pressure as the cleaning agent. It is not. On a weathered fence the gray is oxidized wood fiber, the green is algae and the black is mildew, and all three respond far better to chemistry than to force.
Check the fence is worth cleaning
Before anything else, push a screwdriver into the base of two or three posts right at ground level, where fences rot first. If it goes in easily, the posts are gone and cleaning the boards is decorating a structure that is on its way down. Also check for boards that flex when pushed, nails that have backed out, and rails that have separated from the posts — fix those first, because a pressure washer will find every loose fastener and make it worse.
Then look at what you are actually removing:
| What you see | What it is | What removes it |
|---|---|---|
| Silvery gray surface | UV-oxidized wood fiber | Wood brightener with oxalic acid; pressure alone will not restore color |
| Green film | Algae | Oxygenated cleaner, 10-minute dwell, low-pressure rinse |
| Black speckling | Mildew | Same, with a longer dwell; a sodium percarbonate cleaner works well |
| Dark streaks below fasteners | Tannin reacting with iron | Oxalic acid brightener; pressure makes it worse |
| Peeling stain or paint | Failed coating | Strip chemically; pressure-stripping gouges the wood underneath |
| Soft, punky wood | Rot | Replacement — do not pressure wash it |
Tools and materials
- A pressure washer, ideally an electric in the 1600–2300 PSI class; a gas machine works but you will be running it well below its capability. See the current rankings.
- 25° green tip for washing and black soap tip for applying cleaner. A 40° tip is a sensible extra for softer cedar. A fresh nozzle set is a few dollars and worn tips spray unevenly.
- Deck and fence cleaner — Simple Green Oxy Solve Deck and Fence (paid link) is the most-owned oxygenated option and is formulated to run through a pressure washer's injector.
- A stiff-bristle brush on a pole, a garden hose, plastic sheeting for anything below the fence, eye protection and closed shoes.
- A moisture meter if you are staining afterward, and a wood brightener if the fence is gray.
The method
- Pick the weather. Overcast, above 50°F, no rain forecast for three days, and not in direct sun — cleaner that dries on the wood streaks and stops working. Early morning on a mild, cloudy day is ideal.
- Protect the surroundings. Wet plants and lawn thoroughly with clean water before you start and again afterward; a wet plant dilutes runoff, a dry one absorbs it. Cover anything you do not want speckled, and move the barbecue.
- Pre-rinse at low pressure. A quick wet-down with a 40° tip knocks off cobwebs and loose dirt and stops the cleaner soaking straight into dry wood.
- Apply the cleaner bottom-up. Fit the black soap tip and coat the fence working upward, so runs land on already-wetted wood rather than streaking dry boards.
- Let it dwell about ten minutes. Keep it wet — mist it again if the surface starts to dry. This is where the cleaning actually happens. Agitate stubborn algae with a stiff brush; the brush does more than another 500 PSI would.
- Rinse top-down with the 25° tip. Start at 18 inches back and close to 12 only if needed. Work with the grain, in long horizontal passes along each board, overlapping about 20%. Keep the wand moving constantly; pausing in one place is what tears the grain.
- Keep a consistent distance. The commonest visible fault is a fence with dark and light bands where the operator's arm distance varied. Consistency matters more than pressure.
- Do the posts and rails last, and the far side too if you have access — one clean side and one green side looks worse than doing nothing.
- Brighten if the wood is gray. An oxalic-acid brightener applied after cleaning, then rinsed, restores the warm tone. This is a chemical result that no amount of pressure will produce.
- Let it dry two to three days. Then test: sprinkle water on the boards, and if it soaks in within about ten seconds the wood is ready for stain. If it beads, wait.
The damage that never comes out
Raised grain. Too much pressure, or a tip held too close, tears out the soft spring wood between the harder grain lines. The result is a fuzzy, splintery surface that drinks stain unevenly and weathers faster. On one board you can sand it; on 200 feet of fence you cannot.
Wand marks. Pausing, or a narrow tip, leaves a visible line etched into the board that stain then highlights rather than hides.
Blown-out knots. Loose knots in pine and cedar come straight out under a narrow tip, and you are then patching holes.
Driven-in water. Spraying upward at a horizontal board pushes water behind it and into the rail joints, where it sits and rots. Always angle slightly downward.
Stripped coating, half-off. Pressure-stripping old stain almost never comes out even. Strip chemically or accept that you are now sanding.
Safety
Even a modest machine can break skin. A high-pressure injection injury forces water, grit and bacteria into the tissue beneath through a puncture that looks trivial, and it is a surgical emergency rather than a wound to clean at home. A 2023 analysis in the journal Hand of a decade of US emergency-department data found pressure washers accounted for roughly 90% of high-pressure injection hand injuries, with the index finger the most common site — a pattern consistent with people steadying a wand or clearing a tip with the machine live.
Never point the wand at a person or a pet, never brace a hand in front of the tip, bleed the pressure before changing a nozzle, and never fit the red 0-degree tip on a fence for any reason. Wear eye protection — splinters and grit come back at you at the speed they left — and closed shoes. If a jet breaks your skin, go to an emergency department and say it was a pressure washer injection injury. More: pressure washer injection injuries.
Working off a ladder with a pressure washer is a genuinely bad idea; the recoil is enough to unbalance you. Use an extension wand from the ground instead.
Where to go next
Doing the deck at the same time? The technique is close but the pressure ceiling is lower still — see how to pressure wash a deck. For the machine, the best pressure washers for decks and fences are sized for exactly this work, and the right soap matters more here than the machine does. Siding next? How to pressure wash vinyl siding.
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