The most important external elements that affect the appearance of the wood are fluctuating climatic conditions and weather effects such as sun, rain, wind, snow, polluted air and the like.
To protect the wood from damage caused by various weather factors, wide variety of paints are available. Which coating we can choose for the purpose of our use depends on the desired look in terms of exposure (mountain, sea, urban area ...). One of the most important properties of the coatings for outdoor use, which has besides decorative also the protective function, is the protection of wood against too large humidity.
Too high humidity leads to dimensional instability of wood, tensions in wood, tensions between a coating and wood and tensions in a film itself. Too large humidity causes cracking and peels of the coating, the coat loses its protective function. Temperature changes have influence on shrinking and extension of film, as a consequence they increase the possibility of peels off. Therefore in mountain areas with high temperature differences between a day and night, damages are larger and more frequent.
UV light causes degradation of the surface of the wood and depolymerisation of a film. Frequent rinsing of a film (rainfalls) may reduce its thickness and quality. Smog and various gases from the industrial areas are combined with rain and form acids, which cause corrosion and wear of a film. As soon as first damagaes occur on the coating, we have to renew it, as in this case we provide a long-term protection. Against these damages wood may be effectively protected with combination of construction and surface protection.
A fence with raindrops, that offers a suitable protection.
Legend: 0 – no weather stresses (interior); 1 – minor weather stresses; 2 – medium weather stresses; 3 – intense weather stresses; 4 – very intense weather stresses (direct exposure)
Coatings for surface protection of exterior wood surfaces are generally divided into:
Enamels are film-forming thick-layered coatings, with which we completely cover wood texture and possible defects in wood. They contain a large quantity of pigments and are available in different colour shades. Their main advantage is water repulsion and low vapour permeability, which is at the same time a disadvantage with wood. At the exposure of objects to weather influences, the coating decays, humidity is coming into wood through a film. In such a way there is a »movement of wood«, microcracks appear in the coating film. Too large quantity of humidity in wood is accumulated, which causes blisters and peel off of a coating film.
Lacquers are enamels without pigments, in the past they used them with wood in the interior, when they wanted to keep as natural wood appearance as possible. At the application of lacquers similar problems appear as with enamels. As lacquers do not contain pigments or fillers, it is necessary to protect wood (or its components, which absorb UV radiation) against yellowing and darkening. Usually UV absorbers are added to them. All main wood composites are namely sensitive to UV light, lignin at the most. The result of lignin degradation is the occurence of substance, which are easily rinsed from wood surface. The result of rinsing with water and activation of UV rays is a quicker colouring of surface into grey colour and removal of wood, especially the young one. Thus appears the characteristic relief surface. The degradation level is proportional to exposure time to sun light, at direct exposures it is even more intensive.
Stains are less pigmented coatings, which form a thin film on wood and paint wood surface transparent, so that wood's structure is evident. Stains are water-repulsive, due to high permeability they enable »breathing« to wood. In these cases wood humidity fluctuates much more than at non-permeable or less permeable coatings (enamels). Due to weather influences the stains slowly erode, film thickness gets thinner. The advantage of stains is easy application at renovation. Prior to repeated application sand the surface slightly and wipe it, refresh surface with one or two coats.
To preserve the properties of wood we recommend regular review of all the wooden surfaces. With the normal stress it is sufficient, that you check once a year. You have to pay special attention to horizontal surfaces, as they are much more exposed to weather influences than vertical ones. Even the sky directions have influence on the speed of decay; wood surface on the southern and western sides are more worn out than in the north and east. Particular attention must be given to building furniture and damages caused by hail, as we usually notice them, only when wood on the place of damage gets grey or blue staining appears. In such cases we recommend that you protect the damaged part with stain coating (e.g. transparent). In short the renovation of the complete coating will follow.
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