Prep first
Before you start
- Sweep off debris and dry-brush powdery white deposits before adding any liquid.
- Test any cleaner on a hidden spot first.
- Keep chlorine bleach off pressure-treated wood.
- Never let cleaner or soap dry on the deck.
White marks on a deck are usually mineral deposits or residue left when water dries, though some are damage to the wood's finish that won't simply wash off. Oxygen cleaners are a safe option on this kind of wood, so the mildest clean goes first and a stronger one is only needed if the mark stays. Grayed, weathered wood can be brightened, but fibers that have lost all their natural color stay silver when dry and shouldn't be chased.
A white stain on cedar decking changes a few steps, so check the wood type before you choose a cleaner. Removing white discoloration from an old deck often starts with a hard-water and limescale remover where minerals are the cause. Mildew or dark growth mixed into a white stain on pressure treated wood needs sorting out before you start.
Before you start
What decks & wood fences can take
Safe to use2
- Water
- Oxygen bleach
Use with care5
- Soaking
- Chlorine bleach
- Solvents (alcohol, spot removers)
- Acids (vinegar, lemon)
- Scrubbing and abrasives
7 items
What you'll need
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- Garden hoseFor the rinses and for keeping a section wet during the cleaner's wait time.
- Stiff synthetic-bristle brushScrubs along the grain without leaving iron marks in the wood. (opens Amazon)
- Mild dish detergentThe gentle first scrub, mixed with water. (opens Amazon)
- Oxygen-bleach deck cleanerFor mildew or other organic growth, labeled for wood decks. (opens Amazon)
- Oxalic-acid wood brightenerFor reddish-brown tannin bleed or rust streaks, used per its label. (opens Amazon)
- Plastic drop clothProtects nearby plants from overspray. (opens Amazon)
- Broom and vacuum or dry brushClears leaves and powdery deposits before any liquid goes on.
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6 steps
Step by step
Step 1 of 6
Clear debris and protect plants
Sweep away leaves and debris first. Vacuum or dry-brush any powdery white deposit before you add liquid, and cover nearby plants with a plastic drop cloth. Spray those plants with water to dilute any overspray.
Careful. Water only hides efflorescence for a while, so a rinse won't settle a powdery white deposit.
Step 2 of 6
Test on a hidden spot
Look at where the marks sit and how they appeared, since that tells you whether they're deposits, mildew or damage. Test any cleaner, and any pressure washer setting, on an inconspicuous area before you use it across the deck.
Step 3 of 6
Rinse and scrub along the grain
Wet the boards with a garden hose, then scrub with mild dish detergent mixed in water and a stiff synthetic-bristle brush. Work along the grain, and stay light on cedar, which is a soft wood.
Careful. On an old CCA-treated deck, keep kids and pets away from wet wood, since wetting raises surface arsenic.
Step 4 of 615 minutes
Use oxygen cleaner for mildew
If the marks are mildew or other organic growth, apply an oxygen-bleach deck cleaner in small sections. Keep each section wet for the label's dwell time, about 15 minutes, then scrub along the grain.
Tip. Mist any section that starts to dry with the hose.
Careful. Rinse the oxygen cleaner off completely before any acid brightener, since the brightener neutralizes it.
Step 5 of 6
Rinse the deck thoroughly
Rinse the whole deck with a garden hose until no cleaner or soap is left on the boards. Work section by section so nothing has time to dry on the surface.
Step 6 of 6
Brighten tannin or rust streaks
If you see reddish-brown tannin bleed or rust streaks on the wood, use an oxalic-acid wood brightener and follow its label. Once the streaks lift, rinse the brightener off and let the deck dry.
If you'd rather buy than mix
Best products for white marks on decks & wood fences
Look for an oxygen-bleach deck cleaner and an oxalic-acid wood brightener, both labeled for wood decks, and follow each label. Pick products made for wood rather than a general-purpose bleach.
OxiClean Versatile Stain Remover Powder
How we picked these: each product is made for this type of stain, its maker doesn't warn against using it on decks & wood fences, and its directions fit the steps above.
Finished the steps? How does it look?
Nice work. Let it dry completely and check it again in good light. Here is what done looks like:
How to tell it's really gone →A faint mark usually needs one more round, not a stronger product.
Heavy buildup usually takes more than one round.
Heavy or long-standing buildup →When to call a professional →How to tell it's working
The white should lift as you scrub, and the boards should look even once they dry. Check each spot in daylight after drying before deciding whether anything more is needed.
What changes for yours
Adjust for your decks & wood fences
Cedar decking
Cedar is a soft wood, so the scrubbing stays gentler and the pressure washer stays off. Use only a light pass with the stiff brush along the grain, and skip any harsh caustic cleaner that could strip the surface.
For dark growth on cedar, the same oxygen-bleach cleaner works, but keep it wet for about 15 minutes and rinse it off fully. Chlorine bleach stays off cedar too, so don't reach for it as a faster fix.
Mildew or dark growth
Dark or fuzzy growth is where the oxygen cleaner becomes the main job rather than a backup. Start with the detergent scrub so the cleaner reaches the wood under the growth.
Work one section at a time so each one gets its full wet time. If the dark marks are still there after one pass, repeat the oxygen step on that spot rather than switching to chlorine bleach.
Tannin bleed or rust streaks
Reddish-brown tannin bleed and rust streaks are a separate problem from white marks, and they often show up beside nails. Check the mark's color before you reach for the brightener.
Treat the streaks one board at a time, and rinse the brightener off before the deck is used. Stop once the streaks have lifted, since extra acid adds risk without adding results.
Old CCA-treated decks
An old deck that may have been built with CCA-treated lumber is a different job, and the age of the deck changes the plan. It's worth checking before you start any scrubbing.
Until you know what the boards are, treat the deck as old CCA lumber. A professional can confirm the material before any cleaning starts.
Heavy or long-standing buildup
An old deck that has gone gray and white is usually weathered, and the wood's own color has leached out over time. Commercial deck cleaners, brighteners or restorers can lift the mildew and dirt so the natural color shows again. If the wood still looks silver after that and no natural color is left, leave it alone, because those fibers should stay in place.
Old stain or sealer that won't come off with the gentler steps needs a stain and sealer remover. Neutralize it with a brightener, then lightly sand the boards once they're dry.
If the white marks are still there after all of this, they are likely too deep for a cleaner to reach. At that point, sanding or refinishing the boards is the realistic next step, and a quick clean won't finish the job.
Vinegar, baking soda, peroxide and more
Do home remedies work on white marks on decks & wood fences?
Most household remedies are a poor fit for a wood deck, so cleaners made for exterior wood are the safer pick. The remedies that suit a deck are few, and each one has a specific job.
Works2Skip4
What makes it worse
Mistakes to avoid
Don't use chlorine bleach on the deck
Chlorine bleach damages pressure-treated wood, corrodes metal fasteners, harms plants and doesn't kill mildew spores. It's also dangerous to mix with acid cleaners or ammonia, so keep those well apart.
Don't power wash cedar or soft wood hard
Western redcedar and other soft woods can lose their surface to a power washer, so skip the pressure washer on cedar where you can. If you do use one on wood, choose the lowest pressure that cleans and a fan tip, and spray with the grain, since narrow-stream and rotating tornado tips don't belong on a deck.
Don't let cleaner or soap dry on the boards
Cleaner or soap that dries on the deck can leave its own marks, which look like the white stain you were trying to remove. Rinse as you go so nothing has time to set.
Don't scrub with steel wool or wire brushes
Leftover iron in steel wool or wire bristles reacts with the wood and stains it. A stiff synthetic-bristle brush does the job without leaving marks.
When to call a professional
Call a professional if the deck is old and treated with CCA, since the lumber should be replaced rather than cut or burned, and sawdust and smoke carry arsenic. A deck that stays marked after these steps, or one that a pressure washer has already damaged, is worth a pro's assessment.
Questions
What causes white marks on a pressure-treated deck?
Most are mineral deposits left when water dries, or residue from something else, such as undissolved detergent or road salt. Some are damage rather than deposits, such as moisture trapped in the wood's finish, and those can't simply be washed off.
How can I tell mildew from a mineral mark?
Dark or fuzzy growth points to mildew, while chalky, powdery white marks are usually deposits. Test a cleaner on a hidden spot before you clean the whole deck, since the two need different treatment.
Can I use oxygen cleaner on cedar decking?
Yes, for mildew or other organic growth, the same oxygen-bleach cleaner works on cedar. Keep it wet for about 15 minutes, scrub lightly along the grain, and rinse it off fully.
Is an oxalic-acid brightener the same as bleach?
No. The wood brightener is a different product, made for reddish-brown tannin bleed and rust streaks. Follow its label and rinse it off once the streaks lift.
Will an old, gray deck look new again?
It often gets close. Deck cleaners, brighteners or restorers can bring back the wood's own color, but if all the natural color has leached out, the wood stays silver when dry.
Why does the white mark come back after rinsing?
It's likely efflorescence, a powdery white deposit that water only hides for a while. Vacuum or dry-brush the powder first, since a rinse alone won't settle it.
White Marks on other materials
Other stains on decks & wood fences
Start from the bigger picture
How we know this
Every method, amount and warning here comes from university extension services, industry bodies and product makers. We read them, check every step for unsafe combinations and changed amounts, and write it up so you can follow it.