Wood structures exposed outdoors need protection against the influence of exterior factors such as sunlight and rain. Protection can be achieved with a combination of building design and efficient coating. The ability of a wood surface to accept and hold a coat of paint is determined by the natural characteristics of the wood species and the manufacturing processes used. Natural factors (anatomical, physical, and chemical properties) vary considerably not only between different species, but even within the same species or the same tree.
Typically wood finishing for outdoors lies in the fact that architects as well as homeowners often prefer to use fully transparent or semi-transparent finishes because of the preference of natural color and texture of wood. Because these popular types of finishes (clear finishes, stains) have to transmit some sunlight they contain only small amounts of pigment and may have low moisture resistance capabilities. These products don’t have the requirements for a long and durable finish.
Most wood finishing experts suggest that the wood surface itself needs to be modified to achieve both better photochemical resistance and better dimensional stability. Such treatments or modifications could also reduce other weathering effects such as dark coloring due to mildew. Nonetheless, the finishing products themselves have to be optimized to provide more ultraviolet light protection, less moisture permeability, and more mechanical flexibility. Reasonable design criteria and construction details are also necessary for good durability and performance of both wood and finish.
The primary function of any wood finish is to protect the wood surface, help maintain and enhance the appearance, and provide easier cleaning solutions. Wood surfaces exposed to the weather without any finish will change colour, roughen by the photo degradation process, surface check and erode slowly. Wood surfaces exposed indoors without any finish may change color and accumulate dirt and grease.
Wood and wood based products in a variety of species, grain patterns, textures, and colors can be finished effectively by many different methods. Selection of the finish will depend on the appearance and degree of protection desired, on the substrates used, but, mainly, on the final end use of the product. Since different finishes give varying degrees of protection, the type of finish, its quality, quantity and the application method must be all considered in selecting and planning the finishing and refinishing process of wood and wood products.
There are two types of finishes (or treatments) used to protect wood surfaces exposed outdoors, those that form a film, layer, or coating on the wood surface and those that penetrate the wood surface leaving no distinct layer or coating.
Film forming finishes include:
• Paints of all descriptions
Penetrating finishes include:
• Water repellants
• Pigmented semi transparent stains
• Chemical treatments
From all the finishes, paints provide the best protection for wood against ultraviolet light (UV) degradation and erosion in the outdoor environment. The paint seals the wood against moisture penetration from exterior. Also it seals inside all the wood oils that otherwise would be weathered away. Paint it is not a preservative, therefore it will not prevent decay if the conditions are favorable for fungal growth.
Oil-base or alkyd paints are essentially a suspension of inorganic pigments in an oil or resin vehicle that binds the pigment particles on the bonding agent to the wood surface. Latex paints are suspensions of inorganic pigments and various suspensions of inorganic pigments. Acrylic latex resins are very durable and it is generally accepted that an acrylic paint will outlast the oil-base house paint. Latex paints are also more porous, and the fact that they can breathe slightly contributes to their longevity.
Varnishes are also surface film finishes. These clear finishes have always been popular because they accent the grain and the color of the wood. Unfortunately, all types of varnishes (oil-base, alkyd, urethane, and acrylic) require frequent maintenance to keep up this attractive appearance due to ultraviolet light from the sun, which degrades both the varnish and the wood fibers directly beneath it. Even though new synthetic resins have been made with special UV inhibitors, some UV still gets through the film. Eventually the varnish cracks, peels and flakes off, taking with it the fibers of the photo chemically degraded wood. Cleaning and re-varnishing has to be done as soon as the breakdown occurs.
A large portion off all the damage done to exterior woodwork is a direct result of moisture changes in the wood and, as a result, dimensional instability. The treatments can be also used as natural finishes for wood. Pretreatment for wood with water repellents or water repellent preservatives is very important in the finishing of wood for exterior use.
When inorganic pigments are added to water repellents or other similar transparent wood finishes the mixture is classified as a semitransparent penetrating stain. The addition of the pigment provides color and increases greatly the durability of the finish. These finishes will not peel or blister even if moisture is entering the wood.
Stains can be prepared from both solvent-base resin systems and latex systems. Latex systems do not penetrate wood surface.
The most common used stains are: pigmented oil stain, pigmented latex stain, penetrating oil stain, water stain, spirit stain, non-grain raising stain, shading stain, varnish stain.
Wood Surface Preparation:
Surface sanding has proved to be an advantageous processing step prior to paint application. Sanded surfaces need a relatively low quantity of paint for coverage and show best paint performances even on low-grade wood, which can improve the further performance of wood products.
Wood has always been and continues to be a material of great importance for building and design. It is highly versatile, it is relatively light in weight, yet has good strength in both tension and compression; and provides rigidity, toughness and insulating properties. It can be bent or twisted into special shapes, and it is readily worked, fastened and finished. The finished surface is pleasant to the touch and the visual patterns provided can be of great beauty. Consequently wood is widely used in furniture and windows construction in a variety of ways.
Despite its wide range of application and the profligate use made of it when it is abundant, wood, like most other materials, should not be applied without thought for the conditions under which it will serve or for the inherent properties that will determine its suitability. A natural substance, wood reflects the conditions under which it was grown through the variations in its properties, which vary in different directions. There are marked variations also from one species to another. Consequently there is a continuing challenge in the development of wood technology to recognize the inherent nature of wood and to understand the implications of it at every stage, from the harvesting of the trees to their final conversion to the end uses.
This challenge exists also in varying degrees for the building designer who must know how best to employ wood to achieve the desired results in any given application and how to select and to specify it. It is of overriding importance that the designer should understand fully the anisotropic or directional character of wood, particularly in respect of its strength and moisture response, and to also understand that even high grades of lumber is not completely free of defects within the wood.
Many of the applications and the practices in the use of wood have become established through experience. This basis for predicting performance can be inadequate or even misleading when conditions of use are changed or new applications are involved, unless the reasons for satisfactory past performance can be identified and related to new situations. It is thus important with wood, as with other materials, to understand basic properties and their implications in use in order to be able to select and to design with confidence that the desired results will be obtained.
It is well worth repeating that the outstanding characteristic of wood, apart from its general availability, workability and relatively low cost, is its ability to withstand both tensile and compressive stresses along the grain. It can thus withstand bending loads, and it is this capability that makes it unique among natural structural materials and has always contributed greatly to its value as a natural resource. As this superior strength in tension and compression is along the grain, the length of the structural members obtainable is limited only by the height of the tree or by the length of log that can be handled. The appropriateness of the diameter to length ratio, which often means that the whole trunk can be used as a structural member, is a reflection of the natural function of the trunk in supporting the tree.
The wood materials used in cabinets and furniture are products of nature and include numerous color hues, grain patterns and other natural characteristics. As a manufacturer of wood windows, Unison Windows believes there is a crucial need to educate ourselves and ultimately the final customer in regard to naturally occurring wood characteristics.
Color and grain patterns are the primary factors influencing the appearance of wood. Other characteristics such as stains, burls, and insect damage also have an effect. All are variable, not only in different commercial groups of wood, but within a given species, log, or board. This infinite natural variety is one of wood’s greatest selling points.
Color Variation Influences
The natural causes of color variation within a species are influenced by many factors. Soil types, minerals, water levels, available sunlight, temperature, and genetic composition, all contribute to color variation.
Trees regenerate from seeds, root sprouts, and stump sprouts. Trees originating from seeds contain genetic variables from two parent trees, while sprouts from roots and stumps will be genetically identical to the parent tree. Because of these variables, trees of the same species from one area may be quite different from that of other areas.
The actual color variations are caused by natural chemical extractives found in the cell walls of wood. The hues produced through these deposits cover a wide range and are traceable to four spectral colors: red, orange, yellow, and violet. Other natural influences such as fungi may also contribute to some color variations.
The range of color variation in a commercial lumber group may be increased by the mixing of species, such as northern red oak and pin oak within the red oak commercial group. Logs of these species are normally not separated by sawmills. This mixed lumber will increase the color variation in products manufactured from this material.
Sunlight also slowly attacks and changes the exposed surface of wood, producing slight discoloration from the original color.
Grain Variation Influences
Grain variation, like color variation, is influenced by many factors. They include, but are not limited to, tree size, growth rate, climate changes, site conditions, genetics, bird, insect, and fire damage.