The benefits of coating
Left uncoated, many exposed timbers will dry out which can result in surface checking, warping and hence loss of dimensional stability. Coatings are designed to slow down the rate at which moisture moves in and out of the timber, reducing the stresses on the timber, and thereby reducing the chance for long term splitting, warping and cupping.
Without protection from UV radiation, timber will turn grey over time as a result of the drying and bleaching effects of the sun. For many timbers, this is the first sign of timber degradation. It’s also important to note that timber will not necessarily grey consistently as this can be heavily dependent on orientation and exposure levels.
In cold and humid conditions, constant moisture can result in fungal attack, rot and decay. Coatings effectively seal up the pores and grain in which mould spores can become trapped and grow. Pre-treatments and mould inhibitors such as Intergrain® Mould Killer are recommended in humid and tropical areas as an added measure to prevent mould growth.
Types of coatings
The first distinction to make when selecting an exterior timber coating is whether or not the look of the timber needs to change or whether a natural look is preferred. If the end result needs to look different to the natural appearance of the timber, then a stain is recommended. Containing more pigment than oils, stains are semi-transparent, so they alter the colour while still allowing the grain pattern and knots to show through. Stains are an excellent choice for restoring the colour of aged timber or making a certain species look like one entirely different (e.g. Merbau stain on treated pine, to look like Merbau timber species).
Due to the extra pigment contained in stains, they generally offer greater durability when compared to oil-based coatings. The pigment helps absorb UV light, protecting the timber against exposure and degradation. Stains are available in water-based options such as Intergrain® UltraDeck Timber Stain or oil based with similar distinguishing features as oil-based vs water-based oils.
Water based oils such as Intergrain® UltraDeck Timber Oil are in fact more durable and longer lasting than traditional oil-based coatings and can last up to 3 times longer. This is because water-based oils form a thin invisible film on the surface of the timber that seals the timber and acts as a protective barrier. Oil based products tend to penetrate the surface of the timber more than water-based products but do not form a protective film.
Where odour or access to ventilation is a concern, water-based oils are generally recommended as they are less odorous and emit lower VOCs (Volatile Organic Compounds) and are therefore also kinder to the environment. The convenience of a water clean-up for water-based products is also considered an advantage by some compared to cleaning up with turpentine.
With respect to dry times, water-based oils dry much faster than oil based however the longer ‘open time’ with oil based products is definitely an advantage for large surfaces. Appearance wise, an oil-based coating such as Intergrain® Nature’s Timber Oil will provide a traditional, ‘oiled timber’ look and water based products will provide a slightly higher sheen.
Unlike stains and oils which have varying degrees of pigmentation, exterior clear finishes don’t contain pigment and are generally not as long lasting in comparison. They are typically suited to doors, window frames and trim and are not suitable for heavy wear surfaces such as decking and walkways.
Slip resistance is an important consideration for walkways and ramps or where a standard is required. Additives such as Intergrain® UltraGrip provide a textured surface finish and can be stirred into a coating to meet the required slip rating. ‘R ratings’ are the standard measure of slip resistance and R10 - R12 ratings are generally the most common requirement. It is important to note that a slip resistance additive will cause the appearance of a coating to change, particularly for high gloss products that change to a satin type of finish.
The importance of preparation
Preparing timber properly for coating is important to prevent coating failure, as it ensures good penetration of the coating into the timber surface. New timber must be weathered for 4-6 weeks prior to coating to allow any oils and tannins to leach out of the timber. Presence of oils and tannins can interfere with the drying and adhesion of the coating which can reduce its lifespan.
Old, weathered coatings must be removed by sanding or using paint stripper. For best results, any grey weathered timber should also be removed by sanding back to a fresh surface. A timber cleaner such as Intergrain® UltraPrep Timber Cleaner will help regenerate the colour of greyed timber and will remove oils, contaminants and dry timber fibres which can affect the absorption and adhesion of the coating.
The best way to tell if timber is ready for coating is the water bead test. This involves sprinkling water on the surface of the timber. If the water is absorbed, the timber is ready for coating. If the water beads, repeat the cleaning process with a timber cleaner.
No coating system will last indefinitely as weather and/or foot traffic will wear down the coating over time. It is important that coatings are properly maintained to ensure water is not able to penetrate through any worn areas and come into direct contact with the timber. A 12 monthly inspection is recommended and where areas are looking worn a touch up or re-coat should be applied accordingly. Oil based finishes will generally require re-coating much more regularly than water-based finishes.
Selecting the most suitable type of coating and adequately preparing the timber will help make maintenance easy and will ultimately ensure the longest possible life span of both timber and finish.
Australia is home to the world’s largest accreditation scheme for coatings, the Australian Paint Approval Scheme (APAS), which is managed by the Commonwealth Scientific & Industrial Research Organisation (CSIRO) - arguably the most trusted scientific body in Australia.
APAS certifies that certain paints and coatings meet stringent performance specifications documented in more than 200 APAS performance specifications. APAS itself does not do any testing per se as it is a requirement that it remain independent of testing activities. The APAS List of Approved Products contains more than 2000 approved products and is a comprehensive guide for specifiers.