Application of Coating

The most common application technique for exterior timber finishing is by brush. Although more labour intensive, brushing is a good option for initial coats as it ensures good wetting of the surface and good penetration into grain, joints and corners.

However, in many cases the timber may be purchased pre-primed from the factory. Roller application or airless spray may be used for subsequent exterior coats and is more suited to large areas.

The effectiveness of an exterior paint system depends on building a film of adequate thickness.  Over spreading of paints and stains is a false economy. At all times adhere to the manufacturer’s recommendations with regard to thinning, surface preparation, number and sequence of coats, maximum spreading rates, weather conditions and temperature at times of application an drying, and time interval between coats.

As well as minimum times between coats, the maximum period between coats should be no longer than several days before cleaning the surface to remove any grime or dust that may have built-up. Conditions at the site will obviously influence the extent of this contamination. If the site is subject to salt contamination from prevailing winds off the sea, then washing down daily may be necessary.

The application of stains, particularly semi-transparent types, should be in the direction of the wood grain and completed in one operation (along the weatherboard in a horizontal direction completing the whole length of the board before moving on) if lap marks are to be avoided.

Factory priming and treatment with water repellent-containing preservative of timber components, such as window assemblies, weather and facia board, is designed to protect them from weather and contamination during transport, site storage and an initial period after installation. If in doubt prepare and prime the whole area as soon as possible if not immediately.

Prime cut end grain during installation with two good coats forced into the end grain.

Joinery, cladding and other timber building materials delivered to the site unpainted should be primed all around as soon as practicable. Any cutting or handling that removes primer or leaves uncoated end-grain should be primed before final fixing.

Likewise any primed work that has been allowed to deteriorate through exposure, should be cleaned down and re-primed.

Exterior cladding supplied pre-primed should be fully prepared, primed or undercoated and finishing coats applied within 60 days of installation. Immediately after installation all nail holes and any bare wood should be fully primed; sealants, filler repairs, flashings and putties should be finished.

Some manufactured products that have been primed and undercoated may be left without top coats for longer periods, but priming of bare timber and the completed installation of putties, sealants and flashings is still required.

It is important that any surface to be painted, whether previously painted or not, should be suitably prepared to receive paint.

Painting should not be undertaken during extremely hot weather or when temperatures fall below 10 degrees Celsius. Foggy, misty or damp weather should also be avoided. Work for the day should be stopped early enough to allow for the film to dry sufficiently before adverse conditions develop.

Because of the wide variation in the severity of exposure, variability of timber substrates, the care taken in the initial priming and finishing, and the number of paint coats applied over a period of years, it is impossible to predict the service life of a paint system before re-coating is necessary. Although on average, a period of 5 to 10years may be expected, it is more likely that specific areas, such as those adjacent to timber end-grain, window sills facing north, or sharp arised sections, may need remedial coating before this time. In order to minimise such action, it is advisable to give those areas an additional coat, either primer or top coat at the time of initial painting.

Often in new construction the attention to detail is not always what’s required to meet long term durability expectations. Early maintenance of those sections requiring attention helps significantly in enhancing the performance of the paint coating for an extended period.

The opposite is true for sections of the building where little if any maintenance will be required as those areas are more likely to be south facing and not subject to the same weathering.

The presence of mould and mildew requires removal with fungicidal washes, and rinsing and drying before subsequent coats are applied. If this is not done, mould can spread between coats of paint and result in lifting of the paint film and discolouration.