Wood is made up of microscopically small
cells. The cross section of a piece of wood viewed thru a microscope
looks just like a honeycomb.
No matter where a board is cut from the
tree log, it's surface is made up of cut cells, exposed to the surface
like little "catch basins."
On an actual wooden deck, even though the
cells are treated with an ordinary deck stain or wood preservative, they
are still empty and subject to accumulated air-borne dirt and debris, such
as carbon dust from automotive traffic (brake-pads, tires, and exhaust
emissions,) dust,dirt and tree and plant pollens.
Particles larger than the cells will be
blown or washed away; smaller particles will fall into the cell and accumulate
over time. As the cells fill, we no longer see the color of the wood, but
only the color of the combined particles appearing to the viewer as an
unattractive "grungy gray."
Since vertical surfaces fail to trap as
much soil, less attention need be paid to maintaining the natural color
of the wood. But on decks, the problem is much greater and even accentuated
by the application of"semi-drying" petroleum-based products, such as those
with high paraffin oil content.
Through the application of a penetrating
finish of high solid content which changes from liquid to solid when exposed
to warm air and ultra-violet light, there is little space remaining for
the soil to lodge.
Preserving and restoring the natural color
of the wood, then, is dependent on (a.) getting the surface clean, and
(b.) saturating the wood with a high solid finish. Subsequent applications,
over time, will continue to fill the cells toward the 100% mark. Over-application
is to be avoided, as it will result is a "shiner" or semi-glossy appearance.
When the cells are virtually filled, there remains little or no space for
dirt to lodge, and the wood becomes relatively maintenance-free.
For decks, fences, and siding, etc., penetrating
finishes are superior to coatings such as varnish or polyurethane, because
they do not crack, peel, or become opaque. The application of penetrating
finishes actually strengthens the wood fibers, since the resin residual
is stronger than the cellular structure of the wood to which it is applied.
Super Seal is about the best product for wood decks you will find, a breathable water repellant finish, that allows wood to expand and contract naturally without peeling.
Application should be made as "often as is necessary". On new work, ( See "New Pressure treated lumber") apply it immediately (remembering that
every board has six surfaces exposed to the elements). A thin, second application
at the opening of the next spring season is recommended, then every three
to five years, depending on exposure and cosmetic appearance. At all times,
apply only the amount that the wood will absorb, stretching out the material;
that is, a thin coat is better than a thick one. Any excess (that remains
shiny) after a half hour should be carefully wiped up with a rag.
Related: Cleaning Wood Decks ~ Staining Wood Decks