Protecting and preserving wood

Treated wood
The treated wood – whether that sold from the building supplies department, or that sold in the wood department at Mica stores – is ideal for a range of uses and will provide years of service without failing.

If, however, you require wood to be specifically treated for a particular application then your local Mica will be able to advise you, or refer you to the relevant suppliers of such timbers.

Preserving wood

There are a wide variety of wood preservatives available – ranging from paint, varnishes, sealers and stains to treatments, some of which may be applied at home, but are usually applied by the industry as sold to the consumer.

What we have tried to do here is give you just a few options so that you can select the right preservative for the job at hand.



class: H6
Exposure: Marine
Preservative: CCA plus Creosote
Hazard faced: Marine borers, decay fungi, wood-boring insects above high-water mark

class: H5
Exposure: Fresh water
Preservative: CCA or Creosote
Hazard faced: All of the above, except marine borer

class: H4
Exposure: Outside, in contact with the soil
Preservative: CCA or Creosote
Hazard faced: Decay fungi, termites, wood-boring insects

class: H4
Exposure: Outside, in contact with the soil
Preservative: CCA or Creosote
Hazard faced: Decay fungi, termites, wood-boring insects

class: H3
Exposure: Outside, above the soil
Preservative: CCA or Creosote
Hazard faced: Moderate decay, all wood borers, dry wood termites

class: H2
Exposure: Inside
Preservative: CCA or Creosote or TBTOL or PCPZN (no longer sold through retail channels) or Borate
Hazard faced: Wood borers and dry wood termites


Wood is a natural material – even manufactured woods are essentially natural products – so they are all prone to a be attacked by environmental factors such as direct sun, rain, various types of fungi, and insects.

Fungi feed on wood and cause decay, mould and most sapwood stains. There are essentially two types of fungi: those that destroy the wood by decaying it, and those that stain it – sap-staining fungi. The latter is less of a problem as the wood’s strength is relatively unimpaired, but the staining looks unsightly.


All fungi that grow on wood need a temperature range of between 10-30°C and thrive best in temperatures around 20-25°C. Fungi that destroy wood by causing its decay need the moisture content level to be in the 30% range. Fungi also need oxygen (which is why they cannot attack wood that is saturated) and they need food – the wood itself.


Decay fungi that attack the sapwood and heartwood of a tree appears on wood surfaces as fan-shaped patches of fine, threadlike, cottony growths or as root-like shapes.


Their colour can range from white through light brown, bright yellow, and dark brown. The spore-producing bodies may be mushrooms, shelf-like brackets, or structures with a flattened, crust-like appearance. Fine, threadlike fungal strands grow throughout the wood and digest parts of it as food. In time, the strength of the wood is destroyed.


Decay ceases when the temperature and/or the wood’s moisture content fall outside of the above-mentioned parameters on the fungi’s requirements, but when conditions are favourable again, decay can resume.


Wood decay fungi can be grouped as follows:

  1. Brown rot
  2. White rot
  3. Soft rot


Brown rot breaks down the cellulose component of wood for food, leaving a brown residue of lignin. Brown-rotted wood can be weakened even before decay can be seen. The final stage of wood decay by the brown rot fungi can be identified by:

  1. Cross-grain cracking
  2. Dark brown colour of the wood
  3. Excessive shrinkage
  4. The ease with which the dry wood substance can be crushed to powder with your fingers or broken off.


There are some fungi that are able to conduct water by means of thin strands and they can cause harm to wood that otherwise would be too dry for decay to occur. They sometimes are called the “dry rot fungi” or “water-conducting fungi”.


White rot fungi, which cause the wood to appear lighter than it was previously, act by breaking down both lignin and cellulose.


Soft rot sometimes resembles brown rot and is caused by fungi that usually attack green wood with a high moisture content. The decay begins on the surface and works its way into the wood, causing the wood to gradually soften.


Wood staining fungi

Sap staining fungi penetrate and discolour sapwood, particularly of the soft wood species and as it a penetrating fungus, the stain cannot usually be removed by sanding or planing the wood as the stain occurs as the fungi penetrates the wood. The strength of the wood is not affected to any great extent, but it doesn’t look that good.


Apart from stains caused by fungi, wood can change colour due to chemical staining.

Obviously stains will still be visible when the wood is varnished, sealed or stained, so painting it is a good option.



Several kinds of insects attack living trees, logs, lumber and finished wood products for food and/or shelter. These pests include various termites and beetles.



Termites use wood for food and shelter and are regarded by many as the most destructive of all wood insects.


Dry wood termites enter cracks and crevices in dry, sound wood. Their ability to live in dry wood without direct contact with the soil increases their danger. However, they reproduce slowly and do not destroy wood as quickly as do subterranean termites.



Borers can be a serious pest – and an expensive one. Their larvae bore through wood and over years can create a honeycomb of tunnels within the wood, which can seriously weaken structural timbers, framing members, and other wooden parts of buildings.


Their larvae reduce sapwood to a powdery or sawdust-like consistency – and a give-away sign that borer is probably around are small holes – about a millimetre or so in diameter – in the wood’s surface and powdery deposits adjacent to or on surfaces  below the holes.


Marine borers

Marine borers can do extensive damage to submerged portions of marine pilings; jetty timbers, and wooden boats.


Types of preservatives


Wood preservatives fall into two broad categories: Oil-based and water-based and within each broad category, there is a range of different products, with different formulations and applications. (Note: Not all of the following preservatives might be available in SA, but we have included them so that you know about them should they become available here – and in at least one instance, avoid using the product.)


Paint is a pigmented liquid designed to protect a surface while beautifying it and while individual types of paint contain special ingredients for specific applications (such a fungicide for use on walls in damp environments), they all essentially contain the following: Pigment (for the colour), extender (to add ‘body’), binder (to bind it to the surface) and solvent (water, alcohol, acetate or ketones)

Oil-based paints (also known as alkyd-base paints) protect wood effectively in exterior environments and latex-base paints are also good for interior/exterior use. Water-based paints are also effective but check the labels carefully to ensure that you select the right paint for the environment – interior or exterior. Paints are available is gloss, satin and matte.


Varnishes, like paints, are available in a wide range suitable for every application. Their role is to protect the wood from UV rays, moisture and other conditions that would attack the wood. Many are clear, while others are available in a range of colours, designed to impart, for example, a mahogany appearance to a pine surface.


Oil-based preservatives

These are generally insoluble in water. They are usually dissolved in petroleum or other organic solvents in order to penetrate wood. You can buy oil-based preservatives formulated as water-in-oil emulsions or dispersions-in-water.



  1. Can be dissolved in oils having a wide range in viscosity and colour
  2. Can be glued, depending on the solvent
  3. Ease of handling and use
  4. Low solubility
  5. Toxic to fungi, insects and mould


  1. Can leave an oily, unpaintable surface, depending on the solvent
  2. Dark colour
  3. For some applications, provides somewhat less physical protection to wood than creosote
  4. Oily, unpaintable surface
  5. Should not be used in homes or other living areas because of toxic fumes
  6. Strong odour is toxic and irritating to plants, animals and humans
  7. Tendency to bleed or exude from the wood surface



Creosote is a black or brownish oil made by distilling coal tar.



  1. Toxic to wood-destroying organisms
  2. Relatively insoluble in water and low volatility, which make it very durable under a range of conditions
  3. Easy to apply
  4. Easy to determine its depth of penetration
  5. Low cost
  6. Lengthy record of satisfactory use



  1. Its colour and the fact that creosote-treated wood usually cannot be painted satisfactorily make this preservative unsuitable where appearance is important
  2. Its odour, which some people do not like, and skin and eye irritation – I can vouch for the latter… wiping your brow with the back of even a gloved hand while using creosote on a warm day is not advised
  3. With normal precautions, you should avoid direct skin contact with creosote


Pentachlorophenol (PCP) solutions

Pentachlorophenol is toxic so be extremely careful when handling it or applying it:

  1. Avoid excessive personal contact with the solution or vapour
  2. Do not use indoors or where human, plant, or animal contact is likely


(Pentachlorophenol [due to its toxicity] might no longer be sold commercially through retailers.)


We have mentioned PCP here more as a warning more than anything else… in the very unlikely event that you do happen to come across it, rather select an alternative product. So, be warned.



Copper naphthenate

Copper naphthenate is dark-green, giving this colour to the wood; over time the green gives way to a light brown colour.


Copper naphthenate is effective against wood-destroying fungi and insects.



Chlorothalonil (CTL) [tetrachloroisophthalonitrile] is an organic biocide that is used to a limited extent for mould control in CCA-treated wood. It is effective against wood decay fungi and wood-destroying insects. CTL has limited solubility in organic solvents and very low solubility in water, but it exhibits good stability and leach resistance in wood.


Zinc naphthenate

Zinc naphthenate is similar to copper naphthenate but is less effective in preventing decay from wood-destroying fungi and mildew. It is light in colour and does not impart the characteristic greenish colour of copper naphthenate, but it does have an odour. Water-borne and solvent-borne formulations are available. Zinc naphthenate is not used for pressure treating and is not intended as a stand-alone preservative.


Boron and tributyltin oxide-lindane (TBTOL)

TBTOL does not alter the colour of the wood, while pentachlorophenol (PCP*) and pentachlorophenol-zinc (PCPZN*) may darken the wood slightly.

*See warning above.


Water-based Preservatives


These preservatives include various metallic salts and other compounds. The principal compounds used are combinations of copper, chromium, arsenic and fluoride.

They are often used for lumber, plywood, fence posts, poles, pilings and timbers.



  1. No fire hazard when treating the wood
  2. The wood surface is left clean, paintable and free of objectionable odours
  3. Safe for interior use and treatment of playground equipment
  4. Leach resistant


  1. Unless re-dried after treatment, the wood is subject to warping
  2. Does not protect the wood from excessive weathering



Copper-chrome-arsenic (CCA) is a very widely used heavy-duty preservative that offers insecticidal and fungicidal protection across a broad range of applications.


Copper azole

Copper Azole is a water-based wood preservative that prevents fungal decay and insect attack; it is a fungicide and insecticide.


Borate preservatives

Borate preservatives are odourless and can be applied by spray or brush, are readily soluble in water, but are highly leachable, and so should only be used above ground where the wood is protected from wetting. Subject to these two conditions, borate preservatives are very effective against decay, termites and beetles.


Harmful effects and symptoms

Safety precautions when spraying preservatives:


Preservatives can be harmful. Therefore you should follow the basic safety precautions on the manufacturers’ labels when using them.


Read the label carefully, so you know exactly what you are working with, and what dangers it might present – not only to yourself, but also to pets, plants, aquatic life (if used near a water feature).


  1. Avoid inhaling vapours. Wear a high efficiency half-mask canister or cartridge respirator.
  2. Wear goggles, or a face-piece, and head coverings that are impervious to the treatment solution.
  3. Do not over apply. Always check for leaks in the system if spraying a product.
  4. After every use, wash the goggles and face-piece with detergent and water. Rinse, dry and store in a clean, dry place.
  5. Change cartridges and canisters if you have trouble breathing, or if you smell the preservatives.
  6. Refer to the manufacturer’s instructions on the use and care of the respirator before you use it.
  7. If you spill any of the liquid on your clothing, remove the garment and wash yourself thoroughly. Only then wash and rinse the garment thoroughly – finishing by again washing your hands to remove any product that might have got on to your hands while washing the garment. If the affected area of skin changes -inflamed, numb or in any way gives you cause for concern (blurred vision, for example) then seek medical attention.


First aid in case of accidents

Caution: bear in mind that the following is intended only as a guide and is NOT intended as a substitute for proper medical care by a properly trained and qualified paramedic or medical professional. The golden rule is: if in ANY doubt, seek immediate medical attention.

If any chemical gets into your eyes – flush them with clean running water immediately for at least 15 minutes. Keep the water pressure low. See a doctor as soon as possible.

  1. If you swallow a chemical, get medical attention at once. Drink large quantities of water and induce vomiting. Do not make an unconscious person vomit.
  2. Move anyone overcome by fumes into fresh air. If the victim is not breathing, apply artificial respiration. Call for emergency medical assistance immediately.


Disposal of leftover preservatives

Always refer to the label for disposal instructions, and if you cannot dispose of the leftover solution, contact your local authority for advice. Remember, if the chemical gets into the environment they could do a lot of damage.


Some words of caution:

  1. NEVER use any treated timber in a braai or domestic indoor fire. The product could be harmful or downright dangerous.
  2. If spraying chemicals, do so on a calm day when there is no breeze, and use copious amounts of newspaper (disposed of afterwards) to cover nearby surfaces.
  3. Do not consume anything during the job and make it a habit not to smoke while using products… water-based ones are not flammable, but those that are oil or solvent-based should be treated as flammable. Making it a habit not to smoke to avoid any accidents.
  4. Keep children and pets well away.

These materials are available at Selected Mica Stores. To find out which is your closest Mica and whether or not they stock the items required, please go to, find your store and call them. If your local Mica does not stock exactly what you need they will be able to order it for you or suggest an alternative product or a reputable source.


This owl-house has a green stain for aesthetic purposes, and then a water-based odourless non-toxic preservative to protect the wood.


This kennel has Nutec and exterior plywood front, sides and rear and stand on a pine base, well protected with a waterproof sealant. The roof is waterproof shade-cloth over a ply base – which doesn’t need preservative as it is fully protected.


There is a bewildering range of sealers, preservatives, paints and varnishes on the market. Chat to the department manager for the best advice – and don’t skimp on the finish. (Image courtesy Central Mica Hardware, Fish Hoek


This wood is rotting, as evidenced by the fact that it is easily broken away.


This picket fence employs CCA-treated wood from our local Mica and has stood up to the elements for years. The wood sealer has been applied only because the wife ordered it to be so.


This rustic wheelbarrow, also using CCA-treated wood that has been left to fade over a few years.


The curse of the termite… notice their very different shape to your average ant. (Image: Wikimedia Commons – Public Domain)

imedia Commons – Public Domain)