About Acid Staining Concrete Floors

Acid stains are not like paints. Instead of depositing an even-colored paint film on the surface, they react with the concrete. This produces translucent colors, which show off interesting variations in the surface. In the last few years, acid stains have become very popular.

How Acid Stains Work

Concrete acid stain (also “chemical stain”) is a waterborne solution of metallic salts and acid. Unlike dyes (pigmented stains), acid stains react chemically with the concrete.

The acid opens up the surface by “eating away” particles of cement. The metallic salts then react with hydrated lime (calcium hydroxide) to form insoluble colored compounds. Water from the stain fuels the reaction which usually takes about a month.

Color Variations

The resulting colors are basically variations of only three colors – black, brown, and blue-green. The results 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:

  • 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.

Green concrete – let it first cure for at least 28 days.

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 stain reacts and dries, a layer of residue will form on the surface. After allowing sufficient time for the stain to react, the residue must be thoroughly neutralized and removed. Prepare a base solution of baking soda or sodium bicarbonate (1/2 cup per gallon of water). Pour out on the surface, scrub with a mop and shop vacuum. Repeat with clean water. Once dry, clean with a damp mop.

The surface is now ready for sealing.

Protecting the Acid Stained Finish

One downside of this finish is that 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 from the surface layer of about 1/32 of an inch deep, which now remains very fragile. It would crumble under foot traffic and turn dull quickly.

Contractors apply multiple coats of a sealer, usually acrylic-based. Acrylics are inexpensive but have the softest surface of all sealers. They require maintenance.

The homeowner has to regularly apply a sacrificial material such as wax. 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 Acid Staining

The cost is rather high. The materials include the acid stain and a concrete sealer, as well as neutralizing solution and wax. But the main issue is that it is very labor-intensive.

Just for a simple job, without fancy patterns, contractors charge $2 to $4 per square foot. Much more for a more demanding project.

And there is the on-going cost of regular waxing.

Acid Stained Finishes Do Not Last

Many homeowners are disappointed when, after a couple of 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:

  • Moisture activates the metallic salts, which created the color in the first place, so they continue to react and darken the concrete.
  • Efflorescence, lime and alkalis carried by water dull the color.
  • Transparent sealers needed to protect the surface trap moisture, which activates more efflorescence from the concrete.
  • The stain only penetrates approximately 1/32" and is susceptible to scratches.
  • The surface, which has been partially “eaten away” by acid, is brittle and easily crumbles and wears off. 
  • People forget to regularly wax the surface.

Acid staining damages the surface of the concrete and the acids are not healthy to the applicator or the environment either.

LastiSeal Stain Beats Acid Staining or Painting Concrete

Concrete stains and dyes are replacing acid stains in the decorative concrete market. Read about concrete dyes vs. concrete stains and the lastest concrete coloring methods.

LastiSeal Concrete Stain & Sealer is a break-through in coloring concrete. It combines concrete staining and waterproofing in one step.

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.

The sealer component penetrates 2" to 4" into concrete and waterproofs it permanently.

The color varies widely depending on concrete porosity. The interesting mottled look is similar to acid stained concrete but you can choose from 37 different colors.

The color stain penetrates deeper than acid staining. And any deep scratches are easy to touch up, unlike with acid stains.

It is much more versatile – usable on a wide range of masonry, does not depend on its chemical composition.

Acid staining LastiSeal Concrete Stain
Available colors 3–8 37
Topical sealers yes not required
Fibercrete no check mark
Colored concrete no check mark
Concrete walls no check mark
Concrete blocks no check mark
Bricks and mortar no check mark
Stucco no check mark
Limestone, stones no check mark
Water seepage (deteriorates) waterproofs
Water vapor (deteriorates) reduces
Efflorescence (deteriorates) reduces
Cost high low

Check out detailed information on LastiSeal Concrete Stain & Sealer