a concrete coloring material that produces a unique look whenever
it's applied. No two floors or walls look the same after being treated
with this material. That's the magic of the acid-etch chemical staining
process-an infinite number of possible results limited only by the
creativity of the installer. It's why installers get so involved
with the process, and it's why their customers must have realistic
expectations about the outcome. They should expect variability,
trying to make concrete look interesting, not like some other material,"
says Mike Miller, president of The Concretist, a well-known consultant
to the decorative-concrete industry. He often combines colors from
acid-etch stains, dyes, and tints in his work, and extends the range
even further by applying the stains to concrete already colored
integrally or with color hardeners.
variegated and natural look of the end product is its most attractive
feature, but this variability can also lead to disagreements among
owners, specifiers, and installers. Because so many variables can
affect the final color, contractors should provide an approval sample
of stain applied to a small, concealed part of the concrete to be
Chemical stains can be applied to new or old, plain or colored concrete
surfaces. Although they are often called acid stains, acid isn't
the ingredient that colors the concrete. Metallic salts in an acidic,
waterbased solution react with hydrated time (calcium hydroxide)
in hardened concrete to yield insoluble, colored compounds that
become a permanent part of the concrete. Several companies manufacture
chemical stains that are variations of three basic color groups:
black, brown, and blue-green.
acid in chemical stains opens the top surface of the concrete, allowing
metallic salts to reach the free lime deposits. Water from the stain
solution then fuels the reaction, usually for about a month after
the stain has been applied. Other factors that affect the outcome
* Cement properties and amount
* Admixtures used
* Type of aggregate used Concrete finishing methods
* Concrete age and moisture content when stain is applied
* Weather conditions when stain is applied
general, cements that produce larger amounts of calcium hydroxide
during hydration will show more stain color, and higher cement contents
produce more intense colors. Air-entraining or water-reducing admixtures
don't pose a problem.
calcium-chloride accelerators can cause very mottled, darkened areas,
and for this reason aren't recommended. Nonchloride accelerators
don't cause this mottling effect.
they're near the surface, calcium-based aggregates, such as limestone,
take stain readily and deepen the color of the concrete above them.
Siliceous aggregates, such as gravel, don't react with the stain.
finishes achieved by floating followed by minimal troweling take
more stain and produce denser colors than do hard-troweled surfaces.
However, open finishes lose color faster because the concrete wears
away. Because of this, Gary Jones, president of CP Concrete Systems
in Burnabv, British Columbia, Canada, prefers staining hard-troweled
surfaces because the stain color lasts longer. "Colors on troweled
surfaces also look richer than those on floated surfaces,"
says Jones. "But you have to sand the surface or use a higher
acid concentration to ensure adequate stain penetration."
placed in wet weather result in a richer stain color if the concrete
is stained soon after it's placed. However, wet slabs are more likely
to effloresce, lightening the color and causing a more mottled effect
in areas where the stain doesn't take because efflorescing salts
hinder penetration. On sunny days, the concrete can become hot and
dry, and the stains won't penetrate as deeply into the concrete.
continued presence of water will cause the reaction to continue
for a long time, and concrete stained blue green will gradually
turn brown or even black. Initially, this provides nice variation
to the appearance, but eventually, nearly all the blue-green color
may change to brown and black. Because of the possible color shifts,
some manufacturers advise against using these colors for exterior
concrete. Interior slabs must be placed on a well-drained base or
subgrade and have a low moisture content before stain is applied.
Jones believes the brown-colored "flowering" of blue-
green stains is caused by oxidation of a copper component resulting
from water vapor passing through the slab. Others believe the brown
color is caused by a fungus, which can be eliminated by using sealers
containing a fungicide.
stain finishes don’t require much equipment for application.
For surface preparation, you may need a grinder or a buffing machine
equipped with sanding pads. Power-washing equipment also is useful.
Any equipment that comes in contact with the staining liquid, such
as sprayers, must resist hydrochloric acid. Brushes used to apply
or spread the stain should have acid-resistant, uncolored bristles.
should wear the proper safety equipment including acid-proof gloves,
goggles, boots, and facemasks to filter acid vapors. And good-quality
wet vacuums are highly recommended for cleanup
The work of other trades also can affect staining results. For instance,
drywall dust on a surface to be stained will react with the stain,
coloring the surface differently wherever it's present. And spills
grease and oil. other lime-based materials, paint or caulk. before
or after staining will produce unwanted color variations. A good
relationship with the owner or project manager helps to prevent
such mishaps. Project management should ensure that the floor isn't
damaged before or after staining and keep trades away from areas
where preparation and staining are in progress.
If an installer other than the concrete contractor applies the chemical
stain, the contractor and -installer must agree on the following:
* Pour schedule. To get a rich color, some installers begin
staining with diluted materials as early as a day or two after the
concrete has been placed. For uniform results, they need to stain
each placement at exactly the same age.
* Jointing method. For slabs requiring sawed control joints
or pattern lines, dust or sawing slurry must be removed before any
of it bonds to the slab. Otherwise, stain color at the joints will
differ from the color of the rest of the surface.
* Finishing -process. Although Jones likes to work on a hard-troweled
surface, some installers prefer finishes that more readily accept
the stain. Uniform finishing throughout the job helps to ensure
more uniform stain penetration.
* Curing method. Do not use plastic sheeting, liquid membranes,
or wet curing methods because they can trap moisture and cause efflorescence.
It's often best to install chemical-stain finishes and a first coat
of sealer, and then protect the floor surface with a cover before
allowing other trades on the floor. Unfortunately, any protective
material will affect the final appearance of the stained floor to
some degree, usually by leaving an outline of its shape on the floor
and by darkening the surface a bit. Don't use any cover material
that doesn't allow water vapor to escape. Breathable cloth tarps
are perhaps the best covers for preventing discoloration caused
by the work of other trades.
It has been said, "Every step successfully completed leads
to the suc cessful completion of the next step." This is certainly
the case with chemical-stain work; proper surface preparation is
a vital step.
throw a little water on the surface in several locations to see
if the concrete absorbs water. If it doesn't, curing agents or sealers
may be blocking the entry of stains and must be removed. Also remove
any grease and oil, paint drops, taping compounds, caulk, or other
applying solvents or stripping agents, sanding, and grinding are
the principal removal methods. To pick up contaminants more easily,
use stripping agents that will mix with water. If you choose to
grind the surface, avoid making grinding marks that will reflect
through the colored finish by using either a cup grinding head with
a fine-grit (diamond or black abrasive) or diamond pad. Use a light
touch, laying the cup flat on the concrete and moving it in small-diameter
circles until the blemish disappears.
a slab must be patched, use acrylic-modified, low-shrinkage materials
that will accept stain. These patches will always show in the finished
product, and the owner should be made aware of this.
open up the surface for stain penetration, many installers prefer
to sand floors using floor buffing machines with #60, #80, or #100
paper or screen-mesh sandpaper that allows dust to pass the pad.
This process can add its own pleasing effect to the final appearance
by accentuating high and low areas on the surface. (More material
is removed from high spots, giving them a richer color.)
final preparation step involves carefully washing the surface with
water and detergent. Don't use acid to clean the surface because
it will diminish the effect of the stain. It's best to scrub with
a buffing machine using strip pads (preferably black) and to pick
up the effluent with a good-quality wet vacuum. The surface must
be clean and free of streak marks, footprints, and all residue.
Anything remaining on the surface will affect stain penetration.
Sawing and patterning
cuts and sandblasted patterns achieved with stencils can enhance
the appearance of stained surfaces. Timing of these operations,
though, depends on the desired effect. When you want the overall
stain finish to be as evenly colored as possible, cut lines and
patterns after staining is complete. Stains penetrate differently
around indentations. If there is to be a color change at a pattern
line, cut the line first to form a barrier to stain movement. If
sawed joints are to be grouted, complete the staining and sealing
before grouting to help prevent grout accumulation on the unprotected
lines are generally laid out with pencil or chalk. Mark only where
you cut, and don't use chalk colors that are difficult to remove,
or adhere lines to the concrete surface using clear fixative sprays.
Many tools are available for cutting pattern lines in concrete.
Most installers use grinders or hand-held saws with tables that
ride against guides. Dry-cutting diamond blades that do minimum
damage to the edge of the cut are a good choice. Dust-collection
devices that attach to grinders and saws capture almost all of the
dust. A 1-1/2-inch extruded aluminum "L" angle, available
in most hardware stores, makes a good saw guide.
you cut patterns before staining, cut them just before cleaning
the surface in preparation for the stain. Sawing dust contains free
time that can adhere to the surface, causing color distortion. If
you cut after staining, do it after the first coat of sealer has
effects also can be achieved by applying stencils to surfaces after
staining and then sandblasting to reveal plain or colored concrete
in areas not covered by the stencils. These stencils are usually
made from plastic materials and have adhesive backings that stick
to the floor surface. One coat of sealer is recommended before sandblasting
to improve stencil adhesion.
Stain manufacturers differ on when to apply stain. Some say that
a new slab must cure for 28 days before work is started. Others
suggest 14 days. Installers sometimes prefer to do their work as
soon as possible after the concrete is placed.
choosing how to apply stain, keep the following things in mind:
* Colors are more intense if stain is applied soon
after concrete is placed. Stain diluted with water and applied immediately
can often achieve the same results as full-strength stains applied
* Water drives the chemical-stain reaction. To achieve color
consistency, make sure the moisture content of the concrete is roughly
the same for every placement colored. If one concrete placement
is stained 2 days after it's placed, then other placements should
be stained when they are 2 days old for color consistency.
* Staining, sealing, and covering finished work before other
construction trades return to the area saves on cleanup, achieves
a better-looking installation, and makes damage repair during the
rest of construction easier to handle.
are many ways to apply stain, with each method providing a different
final appearance. However, there are some general guidelines.
often are used to apply stains, but they should be rated for acid
and have no metal parts. Acid will quickly destroy metal parts,
which can affect the color of the stain. Miller advises using a
spray tip with a circular pattern, spraying in a pattern that goes
from left to right and then right to left, with someone scrubbing
the stain into the surface using a medium-bristle brush in a circular
motion just behind the spray. It's important to scrub in the stain
and not just push it around. An additional spray pass just behind
the scrubbing removes brush marks. This method ensures good penetration
and minimal marking from either the sprayer or brush.
water can be used to create different concentrations of stain color.
Wetting the concrete before stain is applied is one way to do this.
Following the application with water from a spray bottle is another
applied by paintbrush will penetrate well, but care must be taken
to minimize brush marks, which are not usually regarded as creative
or desirable effects. Whatever stain application method is used,
be sure to carefully mask surrounding areas to avoid accidental
staining. Acid stains can be difficult, and in some cases impossible,
increased interest in chemical-stain finishes is in the direction
of more subtle effects. Installers fre quently dilute stains with
water to produce less intense effects. For example, one contractor
often applies the stain the day after the concrete is placed, starting
with a 3% stain dilution (3 parts commer
cial stain to 97 parts water by volume), and then adding more acid
to increase the strength to 10%. In this manner, the contractor
can gradually build up color to meet owner expectations. Second
and third colors can also be added in the same fashion to create
a stained overlay is the best solution for concrete surfaces that
show damage or have been abused during construction. Commercially
available overlay materials can be integrally colored, textured,
and stained to provide a new range of decorative possibilities.
The overlays have high flexural strength and wear resistance. As
with everything involving stains, however, it's wise to create a
sample to ensure compatibility of the overlay cement with the stain
and to get owner approval for the result.
dyes and tints
Miller states that using a chemical stain should also involve using
dyes and tints, because the two work hand in hand. Dyes and tints
provide color variations not available in chemical stains, can be
used to treat areas that did not stain well, or can lighten the
are not chemically reactive with concrete, and their appearance
is translucent. They can be organic or inorganic and diluted with
either water or solvents. Jones says dyes are azochromium coloring
agents fine enough to penetrate concrete surfaces, and they can
create bright colors not possible with stain, such as reds and yellows.
Some dyes are UV-resistant, but those that aren't can be coated
with UV-resistant sealers to make them colorfast.
tints, often available in paint stores, are opaque, and the colors
produced can mask deficiencies left by acid stains. Tints can also
lighten the color of the stained surface.
When most of the chemical reaction is complete, a layer of colored
residue with a mildly acidic pH will remain on the surface. This
layer must be thoroughly cleaned off with a scrubbing machine and
water mixed with detergent so the sealer will bond properly when
applied to the surface. Water-based sealers, in particular, have
little tolerance for residual acid. Using pH paper or pencils to
test the surface pH is a good way to ensure that conditions are
the water-and-detergent cleaning mix, add a tablespoon of baking
soda per gallon of water to neutralize any remaining acid. Scrub
with a buffing machine using soft buffing pads (green or white)
or a scrub brush, and pick up the residue with a wet vacuum. Rinse
until the water is clear. Allow the floor to dry 1 or 2 days before
Surface sealers for exterior applications should be acrylic to allow
moisture in the slab to escape. Solvent-based acrylics generally
perform better than water-based products for outdoor use. Silicone-based
penetrating sealers are recommended for applications where a shiny
or wet look is not desirable. A good indoor application consists
of one coat of solvent-based acrylic followed by a topcoat application
of water-based acrylic. maintenance can be performed with additional
applications of water-based acrylic sealers or waxes.
interior slabs, three primary types of sealers are used: acrylics,
urethanes, and epoxies. Acrylics are UV stable, inexpensive, and
easy to apply or re-apply, as necessary. But they have the softest
surface of the three and require the most maintenance. Solvent-based
acrylic sealers are softer than water-based products. They also
provide a wet look that greatly enhances the appearance of colored
sealers are much harder than acrylics. Water-based epoxies bond
well to concrete and provide a clear finish, but they are nonporous
and do not allow trapped moisture to escape. Epoxies are probably
the best choice for concrete countertops and food-preparation areas.
They are not UV-resistant, and reapplication involves more elaborate
preparation than for acrylics.
sealers, though the most costly, provide the most abrasive-resistant
finish. However, they don't bond well to concrete, so they must
be applied over water-based epoxy applications. They are not UV-stable,
and reapplication is expensive.
Construction asked several contractors what they charge to install
acid-etch finishes on floors. Prices vary widely depending on the
part of the country, whether the contractor is union or nonunion,
and the contractor's reputation in the market. The following are
some general price ranges:
-- Simple stain applications that include cleanup and a final sealer
coat and involve minimal slab preparation will cost about $2 to
$4 per square foot. Larger projects typically have the lowest square-foot
-- Stain applications with sawcut patterning and different colors
between sawed lines cost $4 to $10 per square foot.
-- Applications with sawed patterns and multiple color buildups
are priced at about $8 to $15 per square foot.
-- Sandblast stencil work, including stain, cleanup, and sealer
applied only to the stenciled area, runs $12 to $25 per square foot
of stencil area.
Despite the unpredictability of final results, acid-etch finishes
are growing in popularity. In the past, homeowners considered staining
a concrete slab if they didn't know what else to do to improve its
appearance. These days, they are going out of their way to install
concrete so they can have stain finishes. Though stained concrete
floors can be expensive (prices rival those for high-quality ceramic-tile
installations), they are very easy to clean and maintain. Other
popular applications for chemical stains include concrete countertops,
sinks and showers, and plaster stucco walls-both inside and out.
Stains will chemically react with any lime-based material.
terms of color, owners seem to prefer light-tan finishes, which
make up about 60% of the market. Greens and browns are popular,
too, and often work well together. Black washes (1 part stain to
15 parts water) can be used to reduce the contrast between colors.
When talking to customers or specifiers about the possibilities
of chemical staining, ask them to describe the look they have in
mind and to provide color swatches that show variations. Show them
colored photos of completed installations to help in the decision
you are thinking of attempting your first acid-etch staining project,
attend a training seminar. Most of the stain manufacturers and many
retail outlets that sell staining products conduct educational seminars,
often at no cost. Professional training opportunities are also available,
for a fee. •