Deck and wood cleaning – Chemicals, tips and tools

Published On: 16th February 2013

Cleaning and sealing your wood deck is a rewarding but arduous task. In our experience, we’ve seen customers and contractors try to clean their deck by using high pressure washing and nothing else. It’s a mistake that cannot be undone and could damage your deck to the point of replacement. In the following video we demonstrate the difference between using a high quality deck stripper and brightener and not using any cleaners at all. These are the methods we use when providing deck cleaning services in Minneapolis and St Paul, MN.

Why is cleaning and sealing your wood necessary?

Reasons for wood deck cleaning

  1. Contaminants need to be removed before applying deck stain.  Contaminants include dead wood, old or failing sealer, mold, mildew and dirt.  When contaminants are removed new sealers/stain can form a strong bond to the wood fibers and increase the life of the new sealer.
  2. Mold and mildew grow spores in the wood fibers and do not allow the wood on your deck to dry, increasing the chance of rot.  This mold growth and rot is most often seen at the butt ends of the deck boards.  Over time, this rot will cause your deck boards to become brittle and loosely attached to the floor joists.

Reasons for wood deck sealing

  1. Keeps out moisture – Moisture will eventually cause rot but before it does water will wash away natural resins and color in your wood.
  2. Protects UV damage – In our opinion this is one of the biggest reasons for staining your deck.  Prolonged UV damage will break down cell structure causing wavy ridged surfaces, often times leading to large cracks and splinters.
  3. Inhibits mold growth – Nobody wants a moldy deck.  It’s slick, dangerous and your deck can’t breathe so it can’t dry.  Deck stains contain mildewcides, aiding in the prevention of mold growth.
  4. Appearance – A clean and stained deck is much more welcoming than a neglected one.

Damage caused from high pressure washing

Wood is comprised of ridged lignin fibers and soft cellulose fibers.  The lignin is the strength and glue that holds the wood together.  When high pressure washing is used, the cellulose fibers are stripped away from the lignin leaving wavy splintered surfaces.

Wood cleaning chemicals

There is no one solution or chemical that is right for every job.  Knowledge of wood species, sealers and chemical reactions are all factors in determining the best course of action for wood restoration.  For wood restoration its best to go with the rule of conservation which means using the mildest treatment that gives acceptable results.

  • How to clean a wood deckSodium Hydroxide – This chemical is primarily used to emulsify previous coatings of stain, dead wood and dirt.  We often use very diluted amounts to clean bare wood.  When you purchase a pre-formulated stripping solution,  surfactants, buffers and other misc. chemicals are added to work in synergy with NaOH.  Sodium Hydroxide is extremely caustic and will irritate eyes, skin and respiratory system.   Follow the manufacturers guidelines for personal protective equipment (PPE).  Pure Sodium Hydroxide(basic) ranges between 12 and 14 on the pH scale.  Application methods listed below.  To neutralize we apply citric/oxalic acid and rinse.
  • Bleach – You can mix 4 gallons water, 1 gallon bleach (12.5%) and 1 cup TSP (Tri-Sodium Phosphate) for simple deck and wood cleaning.  While a solution like this can be very effective at killing mold and prepping bare wood it has its pitfalls as well.  These include; browning of vegetation (TSP helps to reduce this), loss of natural wood color and respiratory distress.  Bleach’s counter part in wood cleaning, sodium per-carbonate, takes longer dwell times to achieve similar results but is easier to control application.  Because bleach (sodium hypochlorite) is reactive in nature it is more sensitive to dwell time, sun exposure, concentration and surface temp of wood.  So, while bleach may be an effective household chemical for wood cleaning, it takes a seasoned professional to know how and when to use it.  Typically, we use a bleach mixture for maintenance cleaning on wood decks which have already been sealed.  Bleach by itself is basic with a pH around 12.6.  Application methods listed below.  To neutralize this mixture we thoroughly rinse with water.
  • Sodium Per-carbonate –  Advantages to sodium per-carbonate wood cleaners include; ease of use, emulsifies dirt, safe for vegetation, wood color retention and does not cause respiratory distress.  The emulsification of dirt allows for easier low pressure cleaning.  Disadvantages to sodium per-carbonate over household bleach is a slightly higher cost of materials and requires a longer dwell time to effectively kill mold.  In some extreme cases it may not be able to eradicate mold completely.  Sodium per-carbonate by itself is an Alkaline(basic) with a pH around 10.5.  Application instructions below.  To neutralize we apply citric/oxalic acid based brighteners and rinse.
  • Citric Acid – Citric acid is used to neutralize a base or anything above a neutral PH of 7.  If wood is too basic prior to sealing, the new sealer’s curing process will be altered, resulting in premature failure.  If wood is too acidic, it may cause blotchiness and prevent proper penetration of your sealer.  Woods with a lot of yellow, red or brown in them including; redwood, cedar and red oak will tend to have a lot of tannic acid.  Tannic acid combined with iron rich water forms tannin stains.  Oxalic and citric acids reduce the iron compounds helping to suppress the formation of tannin staining.  In summary, acids such as citric or oxalic will brighten, neutralize and remove tannin stains from wood species containing tannic acid.  Application methods listed below.  Pure citric acid has a pH around 2.5.
  • Oxalic Acid – Oxalic acid is faster acting and more aggressive than citric but it’s also toxic.  Citric acid takes a longer time to brighten and achieve pH balance.  Oxalic acid and citric acid can be combined into a wood brightening/neutralizing solution.  A brightening mix with both citric and oxalic acid is how we typically neutralize after applying a base such as Sodium Hydroxide.  Oxalic acid works very well at removing rust stains in wood and concrete.  Oxalic acid by itself has a pH of 1.5.  Application methods listed below.

pH: A measurement of how basic or acidic a substance is.  A pH above 7 is considered basic while a pH lower than 7 is considered an acid.
Caustic: A substance that is corrosive to living tissue. Caustics refer to strong bases, particularly Alkalis.
Alkalis: A water soluble base. The adjective Alkaline is commonly refered to as a soluble base.
Base: A substance with a pH higher than 7. In wood restoration, Alkaline’s are used to emulsify dirt, dead wood, stain and mold.
Acid: A substance with a pH lower than 7. In wood cleaning, acids are brighteners and neutralizers.
Neutralization: A reaction between an acid and a base. Neutral is a 7 on the PH scale. Water is very close to a 7 on the pH scale.

How to apply deck cleaners  

Downstreaming deck cleanersApplying deck cleaner with shurflo pumpApplying deck cleaners with garden pump sprayerWe either use a method called down streaming or we apply the chemicals directly with a custom built battery powered pump.  The application method depends on the toughness of the job.  For more difficult to remove sealers we use our custom pump because it allows a higher concentration of stripper or cleaner.  For transparent and some semi-transparent stains we will downstream the chemicals through our pressure washer.  Downstreaming works by drawing chemical through an attachment at the high pressure supply line.  Most downstream attachments will draw at a ratio of 10-1 to 20-1 (20 gal water to 1 gallon cleaning solution).  Specific pressure washer tips need to be used so the pressure is lowered, allowing the injector to create suction.  Instead of using a battery powered pump to apply cleaners at high concentrations you can use a standard garden pump sprayer or a mop and bucket.

Safety Precautions

Most deck cleaning strippers are harmful if swallowed or come in contact with your skin.  Protect yourself with gloves, eye protection, skin protection and a respirator.  Read the manufactures instructions and safety warnings before starting your deck restoration project.

If you would like to see before and after pictures of completed deck staining projects please visit our deck staining in Minnesota page.


Posted by: Conor OKeefe

14 Responses to “Deck and wood cleaning – Chemicals, tips and tools”

  1. Marilyn Talbott says:

    thank you! I have looked at numerous web sites and found yours to be the most informative and non-biased, giving homeowners excellent information without the high pressure of the salesman! Although I don’t live in your geographic area, I would definitely give you my business if I did! I will recommend your website to others.

  2. Andy says:

    what do you reccomend to cleas cedar railing on a deck the deck is azek but the railing is cedar. do you suggest TSP and oxalix acid or a combination or sometign else. also what ratio of solution to water?

  3. Cindy Epperson says:

    Excellent information. Thank you for sharing.

  4. Tim says:

    Great Information! I have a cedar roof and was wondering what you would recommend using to clean it. I was thinking sodium per-carbonate followed by rinsing and then Oxalic Acid to neutralize.

    I have a pump to aplicate this What should the mixing ratio be and should I add a Surfactant in the SP?
    Will this be hard to rinse off if I can’t walk the roof (from the gutter line) like will it leave a white resedue?
    Will this be enough to clean the Lichen off and moss?

    Thank you for your reply! and all of the information you already put on here

    • Tim says:

      also i was wondering if you have any recommendation where to buy these powders. They sell them on amazon I was wondering if there are better places to order them

      • Kirk Billingsley says:

        You can buy the generic no name 99% pure products on Ebay, Amazon, and others. I bought a 5-pound bag of Oxalic Acid for $14.00 and for $13.00 I purchased the 5 pound size of Sodium Percarbonate.

        This is enough to do my deck every year for five years. I have an IPE Deck and nothing lasts more than a year and yes I have tried IPE OIL, Penofin, Baker and others. I just clean, neutralize , let sit for a couple days and roll, then wipe some Penofin on. I redo the deck one year in fall and the next year in spring. No knots, no splinters and no nail holes.

  5. Karen Darnell says:

    I have a pressured treated wood deck that needs to be cleaned. What do you recommend. I would prefer to use something that is the least toxic.

  6. daniel says:

    I am wanting to wash my desk using low pressure. I have a 3gpm machine, and was curious what the best way to mix chemicals and not have to scrub. Im not worried about vegitation as there is very little around. Deck is over a concrete pad. I would like to down stream the chemical.
    Thank you for any help you can provide.

    • Conor OKeefe says:

      Fill water in a bucket and add your chemical. Mix while you are adding your chemical. Use the low pressure soap nozzle to apply the chemical.

  7. Kirk Billingsley says:

    Oh, and I am in Wisconsin at the same latitude as the gentlemen here with Alltimate. They are spot on for deck maintenance and staining. It’s treacherous up here in the snow belt for decks but if you want your deck to be used for 6 months out of the year then the way they do it is the right way. So many people I talk to rent a power washer, or borrow a friends. Why spend 20k on a deck to then ruin it by using a pressure washer incorrectly. These guys explain the correct way to use one. It should only be used to move contaminants and old stain off the deck with the use of the cleaning and neutralizing process to lift up the old stain, dirt, mildew and other contaminants.

  8. David Rivard says:

    I live in Long Island, where it will remain 55-70 degrees during the day, and upper 40’s to 60 degrees at night for another week or two. I have a 10 year old, 25×25 cedar deck with deck boards in pretty good shape with little visible rot. The deck is on the north side but gets full sun a good portion of the day. I used a stripper and power washer about 2 months ago to remove the old stubbord pealing stain, with plans to stain it in the fall when it got cooler out. The cedar has turned gray since the summer, and has some signs of mold on deck boards (railings are plastic). I need to give the deck a light sanding (going to rent a round wood floor sander) to smooth out areas that got fuzzy from power washing – power washing wasn’t my preference, but the old stain wasn’t coming off. My plan:
    1. scrub on a per-carb cleaner and rinse
    2. wait 12-18 hours or so until dry
    3. sand the entire deck somewhat lightly
    4. scrub on a citric and oxalic mix brightener to bring the PH down and to brighten the wood
    5. wait for 36-48 hours of no rain to dry it out
    6. stain it (using a brush or pad) with an semi-trans or semi-solid oil-based stain that absorbs, like a Ready Seal.

    Overkill? Anything you would change with my plan?
    Thanks so much!

    • Conor OKeefe says:

      I would apply the brightener after the per-carb cleaner. Sand after its had time to dry. We use a square orbital sander. As long as it has an orbital movement you should be fine. Start at 60 grit and you can stop at 80-100 grit. Dont go much higher than that or it will laminate the surface not allowing as much stain to soak in. I’d use armstrong clarkes semi-transparent stain over ready seal.

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