Color Restoration of Redwood Furniture
Redwood is often preferred for outdoor furniture, siding and decks because of its attractive appearance, unsurpassed stability and the durability of its heartwood; however, left natural or with the minimal protection of poorly maintained finishes, any exterior wood's appearance will change color.
There are many causes of wood discoloration. . .
Sunlight, moisture, temperature, microorganisms, dirt and soot
contribute to the discoloration of any wood used outdoors. Effects
can range from wood turning a driftwood gray due to ultraviolet
radiation and precipitation, to it turning nearly black as a result of
mildew growth, soot accumulation or extractive staining. Black
discoloration may also result from iron in nails or other hardware or
from contaminated finishes and airborne particles.
The first step toward a remedy is to identify the source of discoloration.
Using simple household chemicals and a little logic, any homeowner can easily determine the cause of wood discoloration.
Mildew is often a problem on decks and siding in moist environments.
Mildew is a superficial fungus growth that lives on the surface of the wood but does not degrade its structure. Mildew may be found on the moist, north wall of buildings, on shaded decks and in moist areas with a restricted air flow. A relative humidity of 70 percent or more at the wood surface is ideal for mildew growth.
Mildew commonly appears as numerous small dark spots on the
surface of the wood. Gray, fan-shaped areas spread below these
spots as spores from the original colony multiply and are washed
down the surface. Severely infested areas may appear uniformly
gray or black.
To test for mildew, apply a drop of common household bleach (5%
sodium hypochlorite) to the small, black spots in an inconspicuous
area. Mildew spots will disappear within one or two minutes. Dark
spots which are not removed by bleach may be dirt, extractive or
iron stains. To remove a mild case of mildew, scrub the surface
with a mild cleanser or detergent. Next, rinse with household liquid
bleach to kill surviving spores; then rinse with water. For more
severe mildew infestations, use a stiff bristle brush to scrub the
wood with a solution of one cup of trisodium phosphate (TSP), one
cup of household liquid bleach and one gallon of water.
NOTE: Household bleach should NEVER be mixed with detergent
containing ammonia; fumes can be fatal particularly in an unventilated
or enclosed area.
Identification of water soluble extractive stains. . .
Extractive bleeding. Naturally occurring chemical extractives in
redwood are in large part responsible for its beauty, stability and
durability; however, sometimes these water soluble extractives
migrate to the wood's surface. High concentrations cause a discoloration
referred to as extractive bleeding.
Extractive bleeding appears in several forms. Some boards, high in
extractive content, may turn very dark if unprotected by finishes and
exposed to moisture. Extractives may bleed through paints if stainblocking oil or alkyd primers are not used. In other cases, extractives
may discolor the face of paints or other finishes if they are allowed to drip over them. Condensation on the unprimed back side or wicking under the lap from the face of bevel siding are common causes of this.
Removal of extractive stains. . .
Oxalic acid, readily available in paint and hardware stores, is used
to diagnose and remove extractive stains. A solution of oxalic acid
crystals dissolved in water should be applied to a small discolored
area. Extractive stains will fade and disappear in twenty minutes or
less following this application.
To completely remove the extractive stains, wash with one cup of
TSP and one cup of household liquid bleach mixed in a gallon of
water. Rinse thoroughly and follow with an application of four
ounces of oxalic acid crystals dissolved in one gallon of warm water
in a non-metallic container. Apply this to one entire board or
surface area at a time with a soft brush. When the wood dries
thoroughly, rinse with clean water.
Where extractives have bled through a paint or stain film and are
not readily removed by the oxalic acid solution, the only remedy
may be to repaint, using a compatible stain-blocking primer.
CAUTION: Oxalic acid is poisonous but not dangerous if precautions
are taken. CRA recommends that you hire a professional
painter to work with this chemical. If you do the work yourself, wear rubber gloves and be careful not to let the acid or solution
touch your skin or eyes. Wear old clothes. Try not to spill any on
plants. When you have finished, wash containers, clothes and
Common iron nails and hardware can be a source of staining . . .
In the presence of iron and water, the naturally-occurring tannins
in redwood react to form a dark, blue-black iron tannate precipitate.
This reaction occurs in any species of wood with a high tannin content. Long term iron exposure can cause deterioration which appears similar to charring or decay. This is the reason noncorrosive hardware is recommended when building with redwood that will be exposed to the weather. Iron stains may be easy to diagnose because they are often near nail heads, screw heads or other hardware.
Some iron stains may not be directly associated with nails and
hardware. Scattered discrete, dark flecks may result from the use of
steel wool or wire brushes, or from airborne iron dust from machinery.
This type of staining occurs almost immediately after the ironcontaminated redwood becomes wet.
Rough or surfaced green redwood sometimes has inky black marks
where it has come in contact with fork lift blades, rollers, strapping
or other sources of iron. Iron contaminated finishes or water may
also result in a gray-to-black discoloration which is usually uniformly
A two part indicator solution is used to test for the presence of iron
on a wood surface. Apply a 19 percent solution of hydrochloric
acid, followed by a 12 percent aqueous solution of potassium ferrocyanide.
(Available at chemical supply houses.) The formation of a
blue color upon application of these reagents indicates the presence
Most iron stains can be removed by using the oxalic acid solution as
described earlier for removing extractive stains.
Nail stains may be difficult to remove because they penetrate
deeply and it can be difficult to prevent them from recurring. The
following removal method is usually the most satisfactory short of
replacing the nails. First, countersink the nails and swab the holes
with a water repellent. When dry, fill the nail holes with a durable
exterior wood filler compatible with the finish system to be used.
Mortar and cement stains are troublesome. . .
Lime found in mortar and concrete mixes can cause severe
stains when spilled or tracked onto redwood. These black stains
cannot be removed by sanding and no known bleaching agent can
remove them. The only solution in the case of this type of staining
Commercial deck cleaning and brightening solutions. . .
Due to the increasing popularity of wood decks and natural sidings, many commercial products are available for removing discolorations and restoring the new appearance of wood. These products are available in powder or liquid concentrate form and are typically based upon non-chlorine bleaches, detergents and/or oxalic acid.
The advantage of using commercial products is that they are intended
for a specific use, come with comprehensive instructions and
are quite effective. Under normal circumstances, well maintained
decks and siding require only periodic cleaning with such products
to remain good looking.
Powerwashing is fast and effective, but it can also damage wood. . .
Powerwashing has gained wide acceptance as a method of cleaning
and restoring the surface of wood siding and decking prior to
refinishing. Properly performed, there are several advantages to
powerwashing, including savings in time and labor costs.
Remember, though, that water and pressure are fundamental enemies
of wood and that improper powerwashing can do more harm
than good. For best results, hire a professional painting contractor
experienced with powerwashing.
The equipment should be capable of 2000 psi operating pressure
and should deliver a minimum of 4 gallons of water per minute.
Under normal conditions, pressures of 1000 to 1200 psi should not
be exceeded as this can result in erosion of the softer earlywood,
resulting in an uneven, rough surface.
Refinishing is the final step for color restoration. . .
Flashing and caulking should be checked carefully before powerwashing
to prevent moisture from entering the wall cavity behind
the siding. Wait several sunny, dry days after washing before
applying a protective finish.
Once redwood siding or decking has been cleaned and restored, it
is time to apply a protective finish. Remember, high quality
products containing mildewcides, water repellents and ultraviolet
protection provide the best performance. Carefully follow finish
manufacturer's recommendations regarding application conditions,
coverage rates and number of coats.
Natural finishes are not exceptionally durable and may require
reapplication in from one to three years. By maintaining these
finishes on a regular cycle, the amount of preparation prior to
refinishing will be greatly minimized.
Refer to California Redwood Association's Redwood Exterior
Finishes guide for further information on how to protect and maintain
your exterior redwood.