Exterior Timber Stains and Discolouration

The staining of timber and other building materials such as paintwork and concrete is a problem home owners can face when some timbers used are exposed to the weather.

These timber stains are often associated with timber being used in a decking, lining and cladding applications, and are can be caused by a number of reasons. Such staining can be avoided if precautionary steps are taken at the time of building.

If staining has already occurred, it can sometimes be removed from most surfaces using some simple ‘home remedies’. Alternatively professional house cleaners can be employed.

There are a range of different types of wood stains, and remedial treatments:

Types of Stains

Tannin Stains

Wood contains a variety of chemical substances known as extractives, that are soluble in water or alcohol. Tannins make up part of these and, being water soluble, they are easily washed from the timber surface.

When unpainted or unsealed timber is used in situations where it is frequently wet by rain e.g. wall cladding, deck joists and deck flooring, the water runoff may contain tannins which will cause brown stains on brick, concrete or painted surfaces.

Preventative measures include providing some form of protection from the weather or sealing the timber surfaces with paint or other sealing compound. Staining effects will be minimised by using seasoned timber or by careful selection of species. Some species such as Kwila or Merbau are extremely prone to tannin stain.

If painting or sealing is proposed this should be carried out before the timber members are fixed in place, ensuring that end grain and joint areas are well sealed.

Iron Stain

Wood which has a high tannin content, or is preserved with copper compounds is liable to be affected by iron staining when it comes into contact with iron or steel. Iron stains are blue-black in colour and result from unseasoned or wet timber coming into contact with iron based materials such as nails, saw blades, metal strapping or some sandblasting compounds.

To prevent iron staining occurring in timber it is necessary to use seasoned timber and to keep it dry, use stainless steel fasteners and keep iron based metals away from the timber. Sweep the timber surface down once construction is complete, so any residual metal is removed.

Coating the timber with a water repellent product may minimise the incidence of iron staining.

Timber that is constantly being wet by water runoff from an iron surface e.g. an old iron roof, should be coated with a finish which will prevent the iron contained in the runoff from staining the timber surface.

It should be noted that wood preserved with copper based preservatives, such as Alkaline Copper Quaternary (ACQ), Copper Azole (CA) or Copper Chrome Arsenic (CCA) may react with iron based materials to produce iron stain.

Alkaline Stains

Alkaline substances and chemicals such as ammonia can react with timbers high in polyphenolic tannins to produce stains. These resulting stains are normally brown in colour but not as prominent as iron stains.

Adhesives which are strongly alkaline in nature are a common source of staining. To avoid possible staining of timber by products or chemicals which are strongly alkaline, the timber should be protected by some type of physical barrier, ideally paint.

Water Marks

During periods of heavy rain, water can sometimes penetrate the exterior cladding of a building and wet the back of interior wall linings. This can cause water staining on the internal surface of the wall lining as the moisture moves through it. Timber floors can also suffer from water staining if they have been wet. Good building practice will normally prevent any water staining damage on timber surfaces e.g. use of a moisture barrier in exterior wall cavities will prevent any unsightly water marks on internal linings.

Remedial Treatments

There are a range of remedial treatments for the different types of stains.

1. Oxalic Acid

Oxalic acid is a mild bleaching agent and should be used as a 2 per cent solution in either hot distilled water or methylated spirits. The solution should be stored in a glass or earthenware container.

Chemical contact with the skin should be avoided at all times. Rubber gloves should be used when applying the solution and a mask worn when sanding the treated surface.

The solution is easy to apply with a paint brush. Any excess should be wiped off after 15 minutes and the surface allowed to dry overnight. This process may need to be repeated several times.

Once the desired result has been obtained, the surface may be neutralised with a 0.5 per cent borax solution if the acid is likely to affect gluing or finishing of the timber.

If iron staining is the problem, household bleach could be tried before resorting to oxalic acid.

2. Hydrochloric Acid

Dilute hydrochloric acid (10 per cent) may also be used to treat stained timber surfaces. The same precautions applying to oxalic acid should be followed.

3. Sanding

Some surface stains can be removed by sanding the timber surface. This is recommended for damage caused as a result of water staining.

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