Atmospheric Corrosion
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4.8.5.3 Applied Exterior Wood Finishes
There are a variety of finishes that can be applied to wood that is to be
exposed to the weather and atmospheric conditions. The finish selected will
depend on the appearance and degree of protection desired and on the species of wood to be protected. In addition, different finishes provide varying
degrees of protection; therefore, the type, quality, quantity, and application
method of the finish must be considered when selecting and planning the
finishing of wood and wood products. Table 4.24 shows the relative painting
and weathering properties of various woods. The classification of paintability results from the nature of the specific wood. The higher the rating, the
greater the care that must be taken in applying the finishes.
Paints. Paint coatings on wood provide the most protection because they block
the damaging ultraviolet light rays from the sun. They may be either oil based
or latex based. Either type is available in a wide range of colors. Oil or alkyd
paints are borne by an organic solvent, whereas latex-based paints are waterborne. The three primary reasons for using paints are to protect the wood surface from weathering, to conceal certain defects, and for aesthetic purposes.
Paints do not penetrate the wood surface too deeply. A surface film is
formed while obscuring the wood grain. Paints perform best on smooth,
edge-grained lumber of lightweight species. If the wood becomes wet, the
paint film blisters or peels.
A nonporous paint film provides the most protection for wood against
surface erosion and the largest selection of colors of any of the wood finishes. Paint accomplishes this by retarding the penetration of moisture, and
reducing the problem of discoloration by wood extractives, paint peeling,
and warping of the wood. However, paint is not a preservative. It does not
prevent decay if conditions are favorable for fungi growth. Wood preservatives must be used for this purpose.
Water-repellent preservatives. Water-repellent preservatives contain a fungicide or mildewcide (the preservative), a small amount of wax for water
repellence, a resin or drying oil, and a solvent such as mineral spirits or turpentine. A water-repellent preservative can be used as a natural finish. These
preservatives do not usually contain coloring pigments. The type of wood
determines the color of the resulting finish. The mildewcide prevents wood
from darkening.
During the first few years of application, the water-repellent preservative
has a short life. Additional applications are usually required each year. Once
the wood has weathered to a uniform color, the treatments are more durable
and refinishing is required only when the surface starts to become unevenly
colored by fungi.
Special color effects can be achieved by adding inorganic pigments to the
water-repellent preservatives. The addition of pigment to the finish helps
stabilize the color and increase the durability of the finish. Colors that match
the natural color of the wood and extractives are usually preferred.
151
4.8.5.3 Applied Exterior Wood Finishes
There are a variety of finishes that can be applied to wood that is to be
exposed to the weather and atmospheric conditions. The finish selected will
depend on the appearance and degree of protection desired and on the species of wood to be protected. In addition, different finishes provide varying
degrees of protection; therefore, the type, quality, quantity, and application
method of the finish must be considered when selecting and planning the
finishing of wood and wood products. Table 4.24 shows the relative painting
and weathering properties of various woods. The classification of paintability results from the nature of the specific wood. The higher the rating, the
greater the care that must be taken in applying the finishes.
Paints. Paint coatings on wood provide the most protection because they block
the damaging ultraviolet light rays from the sun. They may be either oil based
or latex based. Either type is available in a wide range of colors. Oil or alkyd
paints are borne by an organic solvent, whereas latex-based paints are waterborne. The three primary reasons for using paints are to protect the wood surface from weathering, to conceal certain defects, and for aesthetic purposes.
Paints do not penetrate the wood surface too deeply. A surface film is
formed while obscuring the wood grain. Paints perform best on smooth,
edge-grained lumber of lightweight species. If the wood becomes wet, the
paint film blisters or peels.
A nonporous paint film provides the most protection for wood against
surface erosion and the largest selection of colors of any of the wood finishes. Paint accomplishes this by retarding the penetration of moisture, and
reducing the problem of discoloration by wood extractives, paint peeling,
and warping of the wood. However, paint is not a preservative. It does not
prevent decay if conditions are favorable for fungi growth. Wood preservatives must be used for this purpose.
Water-repellent preservatives. Water-repellent preservatives contain a fungicide or mildewcide (the preservative), a small amount of wax for water
repellence, a resin or drying oil, and a solvent such as mineral spirits or turpentine. A water-repellent preservative can be used as a natural finish. These
preservatives do not usually contain coloring pigments. The type of wood
determines the color of the resulting finish. The mildewcide prevents wood
from darkening.
During the first few years of application, the water-repellent preservative
has a short life. Additional applications are usually required each year. Once
the wood has weathered to a uniform color, the treatments are more durable
and refinishing is required only when the surface starts to become unevenly
colored by fungi.
Special color effects can be achieved by adding inorganic pigments to the
water-repellent preservatives. The addition of pigment to the finish helps
stabilize the color and increase the durability of the finish. Colors that match
the natural color of the wood and extractives are usually preferred.