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WOOD CHARACTERISTICS AFFECTING
COATABILITY
Coatings fall into two categories. Continuous
film coatings are primers, paints, heavy bodied and solid color stains
which form a solid, unbroken film over the surface. Semi-penetrating stains,
the second category, penetrate the surface of the wood to a great extent
resulting in very little surface film. Wood products may be success-fully
coated by one, or both, or neither of these coating types, de-pending on
their characteristics and how they are manufactured.
Wood species are characterized by their
density (weight), presence of earlywood or latewood, their texture (cell
structure) and the extent of extractives including water soluble color,
pitch and oil. Generally, higher density woods (hardwoods) are less suitable
for exterior use than less dense softwoods such as cedar and redwood.
Manufacturing and grading of wood products
also determine their suitability for accepting coatings. Surface textures,
knots or other defects, and moisture content greatly affect coatability.
How the product is cut from the log, to yield edge grain or flat grain
boards, often dictates what type of coating can be used successfully on
the product. Most standard lumber grades contain a high per-centage of
flat grain, whereas premium grades are predominantly edge grain. Flat grain
areas tend to swell and shrink more, are relatively unstable, and will
not hold continuous film coatings nearly as well as the more stable edge
grain surfaces.
CEDAR AND REDWOOD SIDING
Western red cedar and redwood generally
exhibit the best characteristics for holding continuous film coatings.
However, recently being marketed is a grade of cedar siding which comes
from more immature growths and contains a mixture of grains (mixed grain)
involving both flat and edge grain. This siding is rather unstable and
continuous film coatings are a risky choice, even though the product is
marketed under the highly regarded "cedar" designation. Other products
are anticipated as new growths of timber and sub-species are harvested,
and coatings specifiers must remain aware of these new entrants into the
market place.
Water soluble extractives are present in
cedar and redwood, and may leach through coatings as they become activated
by moisture. These may show up as tobacco colored stains, particularly
on light colored stains or paints. They may be controlled with rather good
success by using an acrylic latex stain blocking primer or an exterior
oil based primer before painting, but may cause discoloration on light
colored semi-penetrating or solid color stains.
PLYWOOD PRODUCTS
Exterior plywood, faced with southern yellow
pine, Douglas fir or western red cedar are frequently used for exterior
surfaces, in both smooth and roughhewn surfaces. Smooth sanded plywood
is not recommended for siding, although it is used extensively for soffits
where it is protected from sunlight and moisture. Plywood, especially when
subjected to sunlight and moisture, develops surface checks (cracks).
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These allow moisture to enter,
causing swelling and shrinking resulting in a very unstable surface which
will overstress a paint film, causing cracking and peeling. Alkyd/oil paints
will show early failure. Top quality acrylic paints will usually fair somewhat
better, and are particularly suitable for soffits. However, since flat
grain is predominant in most plywood products, continuous film coatings,
particularly on rough surface plywood, are less suitable than semi-penetrating
coatings.
RECONSTITUTED WOOD PRODUCTS
Hardboard, a relatively heavy type of fiberboard,
is a popular siding product. Continuous film coatings are recommended on
this substrate, not stains. However, water soluble extractives in the product
may leach through paints causing a discoloration. Acrylic latex stain blocking
primers and one or two acrylic latex topcoats are recommended. Pre-primed
hardboard must be primed again on-site before top coating. Storage of the
product under fluctuating moisture conditions will cause a dislodging of
the surface fibers (the primer cannot inhibit this) resulting in an unstable
surface. Re-priming will help stabilize the surface and will help in avoiding
early paint failure.
Other reconstituted products, including
waferboard, strandboard. (OSB) and flakeboard, are difficult to coat successfully
unless the surfaces are treated or overlaid specifically for exterior usage.
In this case continuous film coatings are usually recommended.
WATER-REPELLENT PRE-TREATMENT
The use of a paintable water repellent or
water repellent preservative prior to priming (for continuous film coating)
is highly recommended, particularly for lap or butt joints on siding or
millwork. This will help prevent moisture from wicking into the wood at
these junctures, thus reducing the shrinking and swelling, which over stresses
the coating, resulting in cracking and peeling. Use only products over
which paints can be applied.
PORCH AND DECKING
Decks are subjected to particularly severe
exposure, plus abrasion from foot traffic. The horizontal or flat surfaces
receive maximum sunlight and moisture collection. Resultant swelling and
shrinking of the wood severely overstrikes any continuous film coating,
resulting in cracking and peeling. These coatings are not recommended for
these surfaces. Water repellents and semi-penetrating stains are recommended
coatings. They require frequent refinishing, but it is an easier task than
the sanding and scraping needed to prepare paints or solid stains for refinishing.
Latex products are to be avoided where water can collect on flat surfaces.
PAGE THREE
Exterior Surfaces & Coatings
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