Wood coatings for windows

Durability has always been the first evaluation criterion of exterior- grade wood coatings. With the SAYERLACK HYDROPLUS waterborne coatings, the life of coated wooden windows has considerably increased. These products are designed for the reduction of water absorption and protection from the sun's radiation, as well as for the protection against fungi, and they ensure several advantages in terms of outdoor resistance and solvent emission reduction.
Longer life on outdoor exposure, twice compared to air-drying products. Waterborne wood coatings keep a high elasticity rate in time, unlike solvent-based products, which due to radicalic reactions within the film originated by UV rays, become so brittle over time that they tend to crack. Solvent emission abatement: by 90%. Better quality of life for those who apply these products and drastic pollution reduction. Waterborne coatings are not flammable, can be thinned with tap water, allow fast coating systems without spontaneous combustion and lifting.

Advantages arising respectively from the use of waterborne and alkyd-based products in the manufacture of windows for exteriors.

WATER-BASED WOOD COATINGS
Solvent emission abatement
Not flammable
Equipment can be washed with water
Short drying time
Durability on outdoor exposure
Gloss retention
Non- yellowing film
Resistance to skin formation
Maintains its elasticity on ageing
Lifting resistance (over-coatability)
No spontaneous combustion
Even colour

ALKYD-BASED WOOD COATINGS
Better application conditions
Resistance to cold, fungi and bacteria
Initial gloss
Blocking resistance
Chemical resistance
Limited grain raising
Can be applied on all wood species
Price


The HYDROPLUS clear and pigmented thixotropic Topcoats exhibit good elasticity, no ''blocking'', good vertical hold, excellent flow and good resistance to settling. In order to ensure a good resistance on outdoor exposure, the clear products of the HYDROPLUS series are formulated with an optimal dose of UV absorbers, with the addition of free radical scavengers. While UV absorbers reduce the yellowing and lignin destruction, the UV scavengers increase the outdoor life of the film, since they reduce chalking and gloss loss and increase the UV absorber efficiency. In the case of pigmented products, the choice of high opacity pigments (with a UV filter action), combined with the addition of radical scavengers, has resulted in the highest quality coatings.

The most important precautions to be observed in the use of waterborne coatings are:
1) During application, for both the product and for the substrate and for the environment, a minimum temperature of 15° C should be maintained. Films formed below such temperature exhibit lower mechanical and chemical resistance properties than the standard quality values.
2) Products must be stored in places with a minimum temperature of 5°C.

WOOD SELECTION
The wood must be healthy and without pith and must exhibit some important features. There must be no traces of fungal attack; a slight presence of blue stain fungus is allowed. There must be no traces of insect attack; isolated holes with a maximum diameter of 2 mm on fresh wood are allowed. There must be no transversal cracking. Longitudinal cracking is only allowed if small and/ or fixed with wood. Sapwood can be present when it has features similar to those of heartwood (e. g. Pine); it is not allowed on woods where sapwood and heartwood have very different features. Resin pockets can be present if with a maximum width of 5 mm, fixed with wood and not visible after coating (in case of pigmented systems) or of the same colour (in clear systems).

Danger of resin outflow
Pine, Larch, Douglas are rich in natural resin. It is impossible to eliminate or block it by wood drying. Sooner or later, the heat of sun makes it come out. However, only appearance is affected since protection remains unchanged. Always check the wood quality before use.

JOINERY DESIGN
The edges must be rounded rather than angled at 45°; the profiles must be inclined at 15° and technical solutions must be devised to minimise wood movement, water stagnation and absorption at joints or horizontal components.
BARE SUPPORT PROCESSING
All preliminary mechanical processes on bare wood produce an irregular profile on the surface; to obtain a good coating result, a regular profile is required, and this can be obtained by sanding with increasingly finer abrasive paper, up to 150 for soft woods, up to 180 for hard woods. The relative humidity of wood should range between 12% and 14%, according to the wood species. Wood humidity should always be checked before coating, using a moisture meter.

Wood selection for waterborne pigmented systems
The most suitable woods for outdoor are Hemlock, Fir and Meranti. Pine, Larch and Douglas cause resin outflow, especially close to the nodes according to their origin, to the period the tree has been cut down, and the boards drying. For these reasons, even very highly isolating solvent-based products can ensure that resin will never come out, thereby forming yellow halos. This defect is veryevident with white and much less showy with other colours. In the case of Iroko, Oak and Chestnut, the outflow of tannin compounds (or extracts of various types) causes yellow spots on all or part (for example, only on the jamb) of the joinery. Similar problems occur with Teak, Western Red Cedar and Merbau.

WOOD PRESERVATION
Preservatives are applied to protect wood from fungi and moulds. For this reason, the joinery should always be applied a protective wood stain before coating, both in coloured and pigmented systems. In coloured systems, the applied a protective wood stain also serves to absorb UV radiation through the transparent oxides that colour it. Natural finishing (carried out with clear protective wood stain) are not recommended since they do not contain iron oxides and therefore do not protect the wood as required, degrading very quickly due to the destruction of lignin. Windows treated with protective wood stain only and without Topcoat are not water resistant and the wood may crack. To avoid excessive reduction of the active principles responsible for protecting wood, it is not advisable to dilute the protective wood stain by more than 10% with water. Protective wood stains can be applied by flow-coating or dipping.

End grain protection
When coating exterior joinery, special care must be taken in the treatment of parts where wood is exposed by its end grain. In fact, it is highly absorbant, thus reducing the thickness of the applied coating film, with a consequent lower protection, especially from water (humidity, fog, rain, etc.). Water absorption causes dimensional changes in the wood, which in the end grain zone produce tensions on the coating films, which could crack and lift from the substrate, with permanent damages to the woodwork. To protect the end grain, apply XA 481, waterbased resins sealer with elastic properties, in order to close all open channels. This operation must be carried out before the Topcoat is applied on the protective wood stain.

WATERBORNE TOPCOAT APPLICATION
HYDROPLUS waterborne coatings can be applied by the conventional coating systems (airless, airmix, electrostatic) provided that the equipment is suitable for water contact. Waterbased coatings are also suitable to be used in coating systems where the sprayed material is recovered. Special equipment for water- based coatings is required for electrostatic application. Equipment not in perfect working order (faulty gaskets, too high pressures) can produce considerable defects in the film (e. g. air blisters). The use of a pre- atomiser and/ or of a pre-heater has given excellent results. The first one allows obtaining a better coating atomisation, even at lower pressures, reducing air incorporation and increasing the film transparency. The second one allows better film flow (especially in winter) and a higher Topcoat quality as well as regular results at all times of the year. The pre- heated coating should be between 25°C and 35°C. The equipment must be washed immediately after use. If dry coating films are to be removed, use XA 4060, leaving it for 6- 12 hours, then rinse with water. Do not use the same pump for applying water-and solvent-based products.

Coating thickness
To achieve sufficient outdoor resistance, apply a minimum wet film thickness of 250 micron on the window and 300 micron on shutters. Heavier coats of topcoat should not be applied in a single coat since, especially in the accumulation zones (such as grooves of shaped panels), cracking could be caused by the uneven film drying. Coating thickness should always be checked by means of a thickness gauge.

Iron oxides
The addition of the clear iron oxide Pastes XA 4034/** to the HYDROPLUS clear Topcoats considerably extends the coating life. In fact, they absorb the ultraviolet component of the solar radiation, improving the protection of wood.

DRYING
The drying of waterborne products must take place in rooms with a minimum temperature of 15°C and relative humidity preferably not exceeding 60%. Outside these limits, the drying is slower and the film could exhibit lower hardness and chemical resistance. Drying should always take place in areas with forced air circulation, preferably dehumidified and slightly warm (25-35°C). Let the items cool down before stacking, to prevent blocking.

PACKAGING MATERIALS
Foamed polystyrene, pluri-ball and PVC-based plastic materials are not suitable for packaging items coated with waterborne products. In practice, foamed polyethylene has given excellent results. Given the large variety of materials on the market a preliminary test should always be performed.