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 and often involves working at height, 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 yield big savings in the long-term.
Where to Start?
It is likely that you’re contemplating repainting the exterior woodwork because the existing coatings are starting to fail or the timber itself is in a state of decay. Failure of finishes to exterior timber and damage to woodwork is usually 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. A high moisture content in timber will 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 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 preservative.
Ultra Violet Radiation
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 paint 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.
Exterior paints are categorised as solvent-borne (oil 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.
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 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, especially 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.
Reference and Further Reading
BRE Maintaining paintwork on exterior timber – J Boxall and JA Smith
BRE Digest 422 – Painting Exterior Wood
Architects Journal – Paint, Finishes & Sealants – Brian Keyworth
BS6150: 2006 Painting of Buildings – Code of Practice
BS 5268-5 Structural use of timber – Part 5: Code of practice for the preservative treatment of structural timber
BS EN 927-3 Paints and varnishes – Coating materials and coating systems for exterior wood – Part 3: Natural weathering test