How a Concrete Acid Stain Works
A concrete acid stain is not like a paint. Instead of depositing an even-colored paint film on the surface, they react with the concrete surface. This produces a spectacular variegated or mottled look, which shows off interesting variations of the concrete surface. In the last few years, acid stains have become very popular for commercial and residential concrete floors.
Concrete acid stain is a waterborne solution of metallic salts and acid. The acid opens up the surface by “eating away” particles of cement paste. The metallic salts then react with hydrated lime (calcium hydroxide) in concrete to form insoluble colored compounds.
Acid stains contain corrosive components that attack concrete and they can cause eye or skin irritation and produce strong odors. Personal protection is required.
The resulting colors are basically variations of only three colors – black, brown, and blue-green that produce a limited choice of only about eight hues, mostly tans, browns, terra cottas, and blue-greens. The resulting colors are somewhat unpredictable but that makes it interesting – unless the colors do not turn out well.
The color depends on the surface and the concrete’s chemical composition:
- Due to carbonation, old weathered concrete has very little lime and may not produce intense colors.
- Calcium-based aggregates like limestone react and deepen the color of the concrete.
- Siliceous aggregates like gravel do not react.
- If calcium chloride has been used as a curing accelerator, acid staining causes dark spots and is not recommended.
New concrete – let it first cure for at least 28 days.
Concrete Acid Stain Application
Preparation: Thoroughly clean the concrete. Remove all dirt, oil spots, paints, or sealers.
Spraying and scrubbing: The stain is applied with a low-pressure sprayer. Use a spray tip with a circular pattern, spraying from left to right and then right to left. Your assistant should scrub the stain into the surface using a medium-bristle brush in a circular motion just behind the spray. It is important to scrub the stain into the surface, not just push it around.
Neutralizing and cleaning: As the concrete acid stain reacts and dries, a layer of residue will form on the surface. After allowing sufficient time for the stain to react, the acid residue must be thoroughly neutralized and removed. Prepare a base solution of baking soda (sodium bicarbonate) using 1/2 cup per gallon of water. Pour out on the surface, scrub with a mop and then, shop vacuum. Thoroughly rinse with clean water. Once completely dry, clean with a damp mop.
Protecting the Acid Stained Finish
Acid staining produces a matte finish. The colored surface layer is very thin and must be properly protected and maintained against abrasion.
The acid staining has “eaten away” particles of cement paste from the surface layer (about 1/32″ deep) that becomes very fragile. It would crumble under foot traffic and quickly turn dull.
The surface must be protected with a clear coating which will also provide gloss and enhance the hues. The choice of topical sealers is very wide and includes water- or solvent-based acrylics, urethanes, and epoxies.
Contractors often apply multiple coats of an acrylic sealer. Acrylics are inexpensive and some resist UV-rays fairly well but they have the softest surface of all sealers. Be prepared for frequent, if not annual, re-applications.
Some contractors swear by the multiple applications of floor wax for the best gloss but it has to be regularly re-applied by the homeowner. The problem is that once the enthusiasm wears off, regular waxing is forgotten and then the surface wears off just by foot traffic.
The Cost of Concrete Acid Staining
The cost is rather high. The materials include the acid stain, neutralizing solution, and a topical sealer or floor wax. But the main issue is that it is very labor-intensive. After a proper concrete prep, acid staining, neutralizing the acid residue, rinsing, and drying, the thin stained layer must be protected with a clear coating. It requires regular maintenance – waxing or re-sealing.
For just a simple job without fancy patterns, contractors can charge $3 to $5 per square foot. Much more for a demanding project.
And there is the ongoing cost of regular waxing or re-sealing.
Concrete Acid Stain Finishes Do Not Last
Many homeowners are disappointed when, after several years, the surface color turns dull or dark brown. Acid-stained concrete, when exposed to moisture like in basements or concrete slabs, gradually changes color. Blue-green color will slowly turn brown or even black.
Why concrete acid stains deteriorate:
- Migrating moisture from slabs-on-grade re-activates the metallic salts in the surface so they continue to react and darken the concrete.
- Efflorescence, lime, and alkalis carried by water dull the surface color.
- Transparent sealers needed to protect the surface trap water vapor, which activates more efflorescence from the concrete.
- People forget to regularly wax or re-seal the surface and the brittle surface easily crumbles and wears off.
The stained surface is only approximately 1/32″ deep and is susceptible to scratches, which are difficult to repair. Once the colors fade, you cannot re-stain it again with acid stain! The colored surface has to be ground or sanded off.
LastiSeal Stain Beats Acid Staining!
Concrete stains and dyes are replacing acid stains in the decorative concrete market. Read about concrete dyes vs. concrete stains and the latest concrete coloring methods.
LastiSeal® Concrete Stain & Sealer is a break-through in coloring concrete. It combines concrete staining and waterproofing in one step.
The resulting color varies widely depending on concrete porosity, color, application, and conditions. The spectacular mottled or variegated look is similar to acid staining but in case of LastiSeal, you can choose from 37 different colors!
The materials alone cost much less than acid staining and sealing. And the application is very easy – suitable for do-it-yourself application by homeowners.
Deep scratches are easy to touch up, unlike with acid stains.
It is much more versatile – suitable for a wide range of porous substrates and for vertical surfaces.
LastiSeal Concrete Stain
|Topical clear sealers||required||optional|
|Poured concrete walls||no|
|Bricks and mortar||no|
Check out detailed information on LastiSeal Concrete Stain & Sealer