How to Protect Exterior Woodwork
Protecting External Woodwork in buildings is a constant challenge in hot tropical regions, especially when they are in coastal areas. Heat, Ultra Violet rays and dust; all cause damage to the coating over external woodwork. In case of brittle coating materials like a conventional spirit polish or lacquer; the daily expansion and contraction of wood during day and night, causes the coating layer to develop a net of cracks and makes the coating literally disappear in a few months. But before we go into the topic of what coating is best, lets quickly go through the reason why a coating is required over the external woodwork.
Why do we need Polish/Varnish/Coating on woodwork?
Polish/Varnish is required for two reasons; one is to protect the wood against moisture and the other for aesthetic purpose. Moisture promotes the growth of micro-organisms causing bio-degradation. But that is not all; moisture even causes wood panels to loose shape, resulting in difficulty in opening and closing of window panels and doors. Perfectly polished exterior wood work usually gives in the impression that the building is perfectly maintained. However; protection is more important benefit.
Types of coatings applied (Advantages/Disadvantages)
a) Conventional Spirit Polish/French Polish: In the past; Spirit Polish was used to Polish the Exterior woodwork. Since it was not waterproof thus; even if it does not rain, daily dew was enough to destroy it within 3 to 4 months. Moreover the application required numerous coats adding to the labor cost.
b) Nitrocellulose or NC based Lacquer: Then came the Nitro-Cellulose Lacquer, which is theoretically a waterproof material,but the problem with lacquer is that its layer is brittle. Which means there is no elasticity to cope with the daily expansion and contraction of external woodwork due to temperature changes. Actually, wood has an expansion factor and during the day it expands due to rise in temperature and at night it contracts. This is like the freeze and thaw phenomenon in cold climates with temperature varying on both sides of (-4 C). Constant expansion and contraction of wood creates tiny cracks in the lacquer film/layer and eventually makes the coating disappear.
The per liter cost of Lacquer is less, but with three times the amount of thinner used to apply it, the cost is pretty much same as that of PU or Alkyd based Varnish. Moreover, it requires more expense in terms of labor cost and time.
c) Polyurethane/Alkyd based Varnishes: Coating manufacturers had to come up with a material that is waterproof and able to absorb the expansion factor of wood. They knew; all they require is a coating that has elasticity equal or greater than the expansion factor of wood. They found that Polyurethane and Alkyd based coatings contain this property. Polyurethane/Alkyd based Varnishes are around for many years now and the results are excellent. Some brands also offer Ultra Violet (UV) Protection by adding a specialized chemical called UV absorber. The purpose of this UV absorber is to absorb the Ultra Violet rays emitted by the Sun. Therefore, providing an excellent protective coating, for the exterior woodwork. They cost a little bit more than the lacquer, but infect, costs cheaper in the long run, because of their long life. Usually three or four coats are required with a brush and therefore are much easier to apply, thus; saving time and labor cost.
How to Increase the Life of Varnish on External Woodwork
In order to extend the life of Varnish, the woodwork must be cleaned periodically with a damp cloth to remove dust. What happens; is that the varnish heats up and melts due to intense sunlight in hot tropical regions and is absorbed by the dust present over the surface of woodwork. So, dust should be removed from the external woodwork at least once a week with a wet cloth.
Application of Fresh-up Coat every year:-------- If a fresh-up coat is applied every year, the varnish can last for years and years. No ground preparation like; scraping, staining and multiple coats are required. A fresh up coat can be applied by only a bit of sanding on the surface to make it rough, so that the new coat gets a good grip.
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