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.