Should I pressure wash my wood fence before staining it?

Should you pressure wash a wood fence before staining it?

A wood fence should be cleaned before staining, and a wood cleaner followed by a gentle, low-pressure rinse is the usual way to do it. Stain soaks into open, clean wood. Stain applied over gray fiber, mildew, pollen and dirt wears off with that loose layer instead of holding in the wood.

Pressure is the part that needs care. Each growth ring in a fence board has a band of soft, light earlywood and a band of dense latewood. The USDA Forest Products Laboratory's Wood Handbook explains that power washing cuts into the earlywood faster than the latewood, so an aggressive pass leaves a ribbed surface of raised latewood that is hard to finish evenly. The handbook adds that power washing is gentler on wood than sandblasting, particularly at low pressure, which is why the cleaner, not the stream, should do most of the work.

What does a gray, green or black fence need before stain?

Gray, green and black fences each need a different fix before stain: gray is worn wood, green is usually algae or moss, and black is mildew or iron stain. The Forest Products Laboratory explains that weathered wood turns silvery gray because weathering removes the colored components at the surface. Gray is worn wood, not dirt. A wood cleaner lifts the loose gray fiber, but cleaning does not add color: it reveals the wood's own color, and stain is what puts color back.

Green on a fence is usually algae or moss, and it gathers on the bottom boards, the shaded side and any run under trees. It comes off with a wood cleaner and a gentle rinse. Black has two common causes with different chemistry. Mildew is a fungus whose dark spores show as gray or black specks and films on damp boards. Iron stain is a blue-black mark around nails and hardware. The next section shows how to tell them apart in a minute.

What are the black streaks around fence nails, and how are they removed?

Black streaks around fence nails are usually iron stain, which forms when iron from the fasteners reacts with tannins in the wood. The Wood Handbook names western redcedar, redwood and oak as woods to expect it on when plain steel fasteners are used. It also explains why fences get it and indoor wood does not: iron only moves through wood above about 16 percent moisture content. Pine has little tannin, so on a pressure-treated pine fence a rusting nail leaves a brown rust mark instead.

A quick test separates mildew from iron. Put a drop or two of household bleach on the dark area. Mildew usually bleaches out within seconds. A mark that stays dark is probably dirt, extractive bleed or iron stain. For iron, the handbook recommends oxalic acid, which many commercial wood brighteners contain, applied as a saturated solution of about one pound per gallon of hot water. It turns the iron into a clear compound that must then be washed off thoroughly with water.

Two cautions apply. Oxalic acid is toxic when ingested and can harm plants in high concentration, so wear gloves and eye protection and keep children and pets away until the rinse is done. The fix also lasts only as long as the iron is gone: if traces remain, the stain returns once sunlight breaks the acid down. Under a clear or semitransparent finish, the old coating has to come off before the stain can be treated. Stainless steel fasteners are safe from iron stain, and high-quality hot-dip galvanized ones are far less likely to cause it than plain steel.

Is oxygen bleach or chlorine bleach the better cleaner for a wood fence?

Oxygen bleach is the gentler choice for most wood fences. Many deck and fence cleaners use sodium percarbonate, an oxygen bleach, and the Forest Products Laboratory reports that oxygen bleaches remove mildew and have been reported to be less likely than chlorine bleach to damage wood surfaces, particularly on low-density woods such as western redcedar.

Viance, a maker of wood treatment chemicals, also advises an oxalic acid brightener after a sodium percarbonate cleaner, to restore the wood's natural pH. Products vary widely in strength and additives, so follow the label. When a cleaner claims to restore color, the Laboratory notes the effect may come from washing away a pulped surface, which sanding would also do.

How much pressure can cedar and pressure-treated fence boards take?

Cedar and pressure-treated fence boards should get the lowest pressure that cleans them, delivered through a wide fan tip held well back from the wood. Viance publishes this guidance for treated-wood decks on its TreatedWood.com site. Use a fan tip set between 40 and 60 degrees, never a narrow stream or a rotating tip. Start the spray at least 24 inches away, feather in to about 18 inches, and avoid getting closer than 16 to 20 inches unless the pressure is very low.

The same guidance says to keep the distance steady and work lengthwise with the grain. On a fence, keep the spray off the board tops, bottoms and rail ends, since the Wood Handbook notes that exposed end grain soaks up water. Cedar, a low-density wood, calls for the same care or more, as our guide to soft washing cedar shake siding explains for cedar walls. Our pressure washer PSI chart lists settings for other surfaces around the yard.

How long should a washed fence dry before stain goes on?

A washed fence should dry until the boards pass the dryness test on the stain label, and shaded runs take longest. The Wood Handbook gives the reason behind every such label: nothing will coat wet wood well, and finishing wet wood raises the risk of blistering and peeling. A penetrating stain needs open, dry cells to soak into.

On a fence, the boards that dry last are the ones to test: the bottom boards near grass and sprinkler spray, the post tops and rail ends where end grain soaks up water, and the face that gets no afternoon sun. Brand-new pressure-treated lumber is a special case. The handbook notes that treated lumber is often still wet when it is delivered, and the best course is to wait until it dries and use a penetrating stain rather than a film-forming one. The day count and the moisture-meter check are covered in our guide to stain timing after a deck wash.

What sets the price of fence cleaning and staining?

The price of fence cleaning and staining is set mostly by the fence itself: its length and height, whether one face or both are done, and how much work the wood needs. A lightly gray fence needs a cleaner and a rinse; heavy mildew, iron stain or an old peeling finish adds steps, and stripping a coating adds the most time.

• Size and faces: total board area, counting both faces and the posts and rails if they are included.
• Condition: gray fiber, mildew, iron stain and old finish each add a step.
• Access: shrubs, gardens and a neighbor's side of the fence slow the work.
• Stain type and coats: rough or weathered boards absorb more, and the Wood Handbook notes they can take two coats of semitransparent stain.

Macomb Power Washing Solutions does not publish a fence price, because length, height and condition vary so much. Fence cleaning gets an itemized written quote inside 24 hours and carries no travel fee. It can share a visit with deck cleaning, which starts from $199 at $1 to $2 per square foot.

When should a Michigan fence be cleaned and stained, and how long does the stain last?

A Michigan fence is best cleaned and stained in a dry, warm stretch between late spring and early fall, and the stain then lasts roughly 2 to 8 years if semitransparent or 6 to 15 years if solid-color, depending on the wood. Stain needs dry boards, a dry day inside the label's temperature range and a rain-free spell after, and Michigan's window between cold, wet spring days and the first fall frosts is short. Lawn sprinklers that reach the bottom boards keep them damp every morning, so shut those zones off from wash day until the stain has dried, and if a soaking rain falls in between, wait for the boards to dry again.

Those ranges come from the Wood Handbook's table for vertical wood such as siding, averaged across the contiguous United States, and its solid-color figures assume an ideal three-coat system. Fences take weather on both faces, so expect the shorter end of each range on exposed runs.