Painting Exterior Woodwork
The Preparation Of Exterior Woodwork
The painting and maintenance of external woodwork, is a job often neglected or constantly put off, not because it is necessarily an expensive exercise, but because it can be time-consuming, consisting partly of removing exterior paint with scrapers and sanding, and often involves working at heights, which a lot of people are not comfortable with. However, it’s one of those aspects of home maintenance, where a little time and money spent early on can give big savings in the long-term.
Where Should I Start?
It is likely that you’re contemplating repainting the exterior woodwork because the existing paint is starting to fail, or the wood itself is in a state of dry or wet rot. Often failure of paint to exterior wood, and damage to woodwork can be caused by one or a combination of the following:
Fluctuations in moisture content will cause expansion and contraction in timber, raising of the grain, splitting and distortion. High moisture content in timber can cause paint films to lose adhesion and fail and will also cause wood to rot.
The end-grain of joinery is most susceptible to the absorption of water and is often the most neglected area. It’s important, therefore, to ensure that end-grains are fully sealed and this includes not just obvious area such at the end of window sills, but on internal joints & in the window reveals as well.
If the moisture content in wood remains over 22% for a prolonged period of time, then micro-organisms such as fungal spores can germinate and lead to wet rot. It is essential that any new timber is treated with a suitable wood preservative.
Direct sunlight will cause wood surfaces to deteriorate. Bare timber left exposed to sunlight over a period of time, will degrade and reduce its ability to hold a painted surface. Varnishes tend to fail early because they do not filter out the radiation. Dark paint colours will also absorb more radiation and should be avoided in south-facing areas.
Existing paintwork in reasonable condition can be simply rubbed down with a good quality abrasive. Any paintwork which is loose or in bad condition will need to be removed by either scraping or with a heat gun back to a firm edge. It isn’t necessary to remove paint which is in good condition.
Wood which has been weather damaged (usually grey in appearance) will need to be sanded back to a new surface if possible. Treat any bare wood with a suitable preservative and allow to dry before priming.
If you need to make good any damaged woodwork always use a two-pack epoxy based filler for medium to large areas. Mix small amounts of filler and build up in two or 3 layers rather than trying to do the job with one lot. Only use ready-mixed fillers for very small areas or for surface imperfections.
Types Of Paint Systems
Exterior paints are categorised as solvent-oil-based based) or water-borne (water-based or ‘quick drying’). Oil-based systems are generally based on a separate primer/undercoat and finish while water-based systems tend to be a coat-on-coat or ‘all in one solution’.
In terms of finish, the oil-based systems will give a high sheen or glossier finish but in terms of performance, the low-sheen water-based alternatives can perform better.
The Building Research Establishment
In 1984 the Building Research Establishment (BRE) conducted a series of trials to determine the performance of exterior paint systems for external timber. Surprisingly they found that water-based paints systems performed significantly better over time than conventional oil-based paints and low sheen finishes performed the best.
Even more surprisingly they found that a traditional system of one coat oil based primer with one undercoat and one coat of gloss performed the worst of all.
Paints described as microporous, moisture permeable or breathable claim to allow water trapped within timber to escape as a vapour and so improve the performance of the paint systems. There is, however, little evidence to suggest that ‘breathability’ is the major factor at work. These systems do indeed perform well but mainly because they have been formulated for exterior use with a balance of properties including increased flexibility, adhesion and fungal resistance.
The paint system you opt for will usually be determined by convenience and/or your budget. Water-based paints are generally easier to use and can be re-coated after a few hours. Oil-based paints will give a traditional glossy finish but can be time-consuming and require more thorough preparation.
Premium branded paints, such as Dulux Weathershield, Sadolin Supadec, are specially formulated for exterior use, & will always perform best, but can be costly compared to traditional alternatives. However, it will often be a false economy to try to save money on materials because although the initial result may be comparable it will often mean you have to repaint more often.
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