How to clean mold and mildew off a wooden swing set

The short answer

Test first, then wash. Put a drop or two of fresh household bleach on a dark patch. Mildew will usually lighten in 1 to 2 minutes. A stain that does not lighten is probably dirt, tannin bleeding from the wood, or iron stain²¹.

If it is mildew, wash it with a weak bleach mix:

• Mix: 1 part bleach to 5 parts warm water should be enough. Never go stronger than 1 part to 3².
• Detergent: a little powdered detergent. Never one that contains ammonia²³.
• Wait: let it sit 10 to 15 minutes so the bleach reaches into the cracks⁴.
• Scrub and rinse: a soft bristle brush or sponge, then a garden hose²⁴.
• Dry: at least 2 warm, windy days before you stain or seal³.

Skip TSP (trisodium phosphate), because the coatings committee warns that its phosphate may feed new mildew⁴. Skip the high pressure washer on soft cedar too, for the reasons below.

What mildew on a swing set is

Mildew is mold growing on the surface of wood or a finish. Most people call it mold on a playset, and here the two words mean the same thing. The Forest Products Laboratory (FPL), the US Forest Service's wood research lab, uses the word for both the fungus and the stain it leaves¹.

It is usually black, though it can be brown, red or green, and green is often algae growing alongside it¹². It grows best at about 70 to 90 F, and below freezing it goes dormant but does not die⁴. The FPL says it is most common where air does not move and dew settles, such as behind trees and shrubs². That is why it returns each spring on the shady side of a set.

What it does not do is eat the wood. Mildew cannot digest the cellulose and lignin that give wood its strength, so it lives on the oils and extractives instead⁴. The 2010 Wood Handbook says these organisms "cannot decay wood"², and the coatings committee says mildew "does not decrease the structural integrity of the wood"⁴. Rot is a different fungus, covered in timber rot and structural repair. The catch is that the same damp, shaded spots suit both. The FPL says decks left without upkeep develop checks, raised grain and mildew, "thus increasing the risk of decay and insect attack"³.

Five recipes, side by side

Every source with a recipe uses household bleach, detergent and water, but they disagree on strength and on the kind of detergent.

Two things settle the spread.

Strength. The oldest recipe is the strongest, up to about 1 to 1. The Wood Handbook, the FPL's later word, says to use "as dilute a solution as possible"². Its ceiling of 1 to 3 is also the CedarWorks and 2013 deck mix⁶³. So start at 1 to 5 and step up only if the spots stay.

Detergent. The coatings committee wanted liquid dish detergent because most powders do not rinse off easily in cold water⁴. The FPL says no liquid, because it "may contain ingredients that react with bleach to give toxic fumes"². Both are guarding against ammonia. Follow the FPL and use powder in warm water, as its own recipe does.

Vinegar or bleach: why the advice splits

Home pages split both ways, and neither side is about a playset. Ideal Home says not to use "any type of bleach" on wood, because its water "can stimulate the regrowth of mould". It recommends a vinegar and water spray left for an hour. The same page later says to avoid vinegar on untreated wood⁹. Bob Vila goes the other way: 1 part detergent, 10 parts bleach and 20 parts water, left to air dry without a rinse¹⁰. That is 1 part bleach to 2 of water, stronger than the Wood Handbook's 1 to 3 ceiling, and the Handbook says to rinse thoroughly².

The federal and industry sources settle it for bleach. The FPL and the coatings committee both publish bleach recipes for outdoor wood, and both say mildew must be killed before you refinish, or it "will grow through the new paint"¹⁴. None of the sources read for this page gives a vinegar method for wood. Ideal Home's worry about water is fair, but the FPL's answer is drying, not skipping the wash: rinse, then allow 2 warm, windy days³. For mildew you are about to seal over, use the bleach mix. If you try vinegar anyway, keep it out of the bleach bucket, since the 1991 FPL report says to be "extremely careful" what cleaner goes in with bleach¹.

The FPL does name a gentler option: sodium percarbonate, an oxygen bleach used in many deck cleaners. The Wood Handbook says oxygen bleaches remove mildew and "have been reported" to harm wood less than chlorine bleach, especially soft woods such as western redcedar and redwood. It also says the two are hard to compare². Gorilla simply says to use an "outdoor wood cleaner"⁷.

What the EPA's caution about bleach means here

The EPA's mold guide says bleach "is not recommended as a routine practice during mold cleanup"⁵. That sounds like it overrules the FPL, but the two are answering different jobs. The EPA guide is written for mold inside a home, where its routine is detergent, water and fixing the leak. Its line for a do-it-yourself job, a patch under about 10 sq ft, is for indoor mold too⁵. The FPL's bleach is not routine. It is a step before refinishing, because a new coat over live mildew fails¹⁴. So use both rules:

• Spring wash, no new coat this year: detergent, water and a soft brush, as the EPA describes⁵. The coatings committee warns the mildew will come back if the shade and damp stay⁴.
• Mildew you are about to stain or seal over: the bleach mix above.

Take the EPA's safety advice either way: gloves of natural rubber, neoprene, nitrile, polyurethane or PVC with bleach, goggles without vent holes, and an N-95 if you want to limit the spores you breathe. Scrub and rinse rather than just spraying, because "dead mold may still cause allergic reactions"⁵.

Power washing a wooden set

The coatings committee in 1999 named a power washer as the first choice and hand scrubbing as the second⁴. One playset manual still lists "power washing, sanding or other preparatory methods" before sealing⁸.

The FPL's later papers are more careful. Power washing "can remove wood from the surface, particularly on softer wood such as western redcedar"³. It wears the soft bands of each growth ring faster than the hard ones, leaving ridges that are hard to finish. If you use one, the Wood Handbook says to "keep the pressure low" and point the stream down².

No source here gives a safe pressure figure. The only number is a warning: the 1991 FPL report says stripping paint with water takes about 600 to 2,000 psi, and that the water "can erode the wood"¹. Gorilla and CedarWorks both build in cedar⁷⁶. On a set like that, scrub by hand. Cleaning and sanitizing explains why the furred grain left behind holds water and splinters.

A shaded cedar set, worked through

Take a 10 year old cedar set under a maple, last sealed three years ago, and suppose the north side of the fort and the swing beam have gone black.

1. Test. Drip fresh bleach on a black deck board and on a dark ring around a bolt head. Suppose the board lightens in two minutes and the ring does not. The board is mildew, and a blue-black ring at a fastener on cedar fits the FPL's picture of iron stain².
2. Probe. Push an awl into the post at ground level. If it sinks with light hand pressure, stop: that is a rot repair, not a cleaning job.
3. Prepare. Take off swing seats and anything made of rope; the cleaning and sanitizing page explains why bleach stays off rope. Protect the plants below, as the coatings committee advises⁴.
4. Mix. For about 3 gallons, use 2 qt of bleach in 2-1/2 gallons of warm water, with about 1/4 cup of powdered detergent. That is 1 to 5, half the Wood Handbook's batch².
5. Wash. Work one face at a time. Wet it, wait 10 to 15 minutes, scrub with a soft brush along the grain, and rinse⁴².
6. Step up only if needed. If black is still there after the rinse, mix 1 to 3 for that patch. Do not go stronger².
7. Treat the rings separately. The Wood Handbook says an oxalic acid brightener removes iron stain and tannin bleed, followed by a warm water rinse. Oxalic acid "is not effective for removing mildew", so it comes second, not first².
8. Dry and seal. Allow at least 2 warm, windy days³, then do the water drop test. Seal with a finish that contains a mildewcide, a mold-killing additive, as the Wood Handbook advises after a mildew wash². Keep children off for 24 hours after it goes on⁷.

Painting and refinishing covers the coat that follows.

What to do with what you find

Where this stops applying

This page covers mildew on the bare or stained wood of a backyard set. Mildew under paint or a clear finish is shielded by that coat, which the Wood Handbook says makes it hard even to test or clean². The 1991 report and the coatings committee say the whole coat then has to come off¹⁴. If that paint went on before 1978, read lead paint on playground equipment before you sand anything. Sets built from CCA treated wood have their own handling advice. Bleach is also not a general disinfectant for everything an animal leaves: it does not kill raccoon roundworm eggs. The FPL recipes come from siding and deck research, not from playset tests. Your own manual decides the warranty, so if it names a cleaner or forbids one, follow it. The EPA's health guidance on mold is general, and a child with asthma or a mold allergy is a question for their doctor⁵.

What to do next

Do the drop test on the darkest board, and if the finish is due, plan the wash for a warm spell with two dry days after it. Backyard annual maintenance sets the recoat cycle, and how long to wait to stain a new swing set explains the water drop test used before sealing. While the wood is clean, look for checks along the grain: cracks in swing set wood sorts the normal ones from the ones that mean stop.