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Acid Staining Concrete – Tips for Success

Concrete acid stain can be applied to new or old, plain or colored concrete surfaces. They are available in 10 basic colors. Although they are often called acid stains, acid is not the ingredient that colors the concrete. Metallic salts in an acidic, water based solution react with the concrete to permanently color of the surface. Siliceous aggregates such as gravel or sand, do not react with the stain. Surfaces containing a higher content of cement will react more than one with less cement yielding more intense colors.

Since each surface is different, results may vary from one surface to another. We always recommend testing a small area to determine how the final result will appear. Note that the final color will not be apparent until the sealer has been applied. With wood stain you can still see the grain of the wood through the stain, acid stain is very similar to wood stain as you can see all defects or finishing marks through it.

Factors that may affect the final results include:

  • Cement properties and amount used, Admixtures and type of aggregate used.
  • Concrete finishing methods, concrete age, and moisture content
  • Porosity and Texture of the surface
  • Weather conditions when the stain is applied
  • Efflorescence.

Concrete acid stain finishes do not require much equipment for the application. The applicator usually uses a garden sprayer that is completely plastic. Some prefer a fine bristle brush or a combination of both. All equipment that will come in contact with the acid stain, such as sprayers, must be resistant to acid. Brushes to apply or spread the stain must be resistant to acid, and also colorless bristles.

Workers must have the proper safety equipment, including acid-resistant gloves, goggles, boots and masks to filter the acid fumes. A good quality wet vacuum is recommended for cleaning. Golf spikes are also recommended because footprints will show through and create undesirable markings in the final appearance.

Surface Preparation

Surface preparation is considered the most important step in any decorative concrete application. It is important for the immediate and long term performance of all decorative concrete applications. Poor surface preparation can turn a simple process in to a difficult and lengthy repair.

First you will need to throw some water on the surface in several places to see if the concrete accepts the water. If it is not absorbed by concrete, you may have a sealer on your surface. If so, you will need to strip the surface using EnduraPrep Coatings Stripper. This stripper is ideal for removing coatings such as paint or acrylic stains and colorants. If there is not a sealer on the surface, but it will not absorb water, then your surface is too dense. This is usually caused by over troweling of the surface when the concrete was poured. It is very important to condition these types of surfaces to accept the acid stain. Surface conditioning is often the key to success. If it does not accept the water, it will certainly have to be conditioned. Sometimes, very dense surfaces must be conditioned twice. When the surface is conditioned properly, it should feel like sandpaper of 120 grit. To use EnduraColor Reactive Concrete Stain, DO NOT USE hydrochloric acid on the surface because it would deprive the concrete of the necessary minerals to react with the acid stain. A properly conditioned surface can be easily accomplished using EnduraPrep Concrete Surface Conditioner, a safe alternative to the typical acid etching.

Finally, we will wash the surface with EnduraPrep Cleaner and Degreaser. The surface must be clean and free of grease and oil, drops of paint, taping adhesive residue, caulk, cement, or any other surface contaminants. All that remains on the surface will affect the final result of the surface. EnduraPrep Cleaner and Degreaser is excellent for cleaning any surface contaminants. You can dilute 10 to 1 for cleaning your surface and use more potent for persistent surface contaminants.

If patching is necessary, you should use a material with low shrinkage that will accept the stain. The final result will always show these patches. The owner must be aware of this.

Decorative patterns with templates can improve the appearance of stained surfaces. The timing of these operations, however, depends on the desired effect. When you want the final appearance to be as even as possible in color, cut lines and patterns after staining is complete. Stains penetrate differently around cuts and indentations. If you want there to be a color change at a pattern line, cut the line first to form a barrier to stain movement. If sawed joints will be grouted, complete the staining and sealing before grouting to help prevent the stain from coloring your grout.

Patterns are usually arranged in pencil or chalk. Mark only where you will cut. Also don’t use chalk that is difficult to remove. Many tools are available for cutting pattern lines in concrete. Most installers use grinders and hand saws with tables, riding against a guide. A 1-1/2 “extruded aluminum” L “angle, available in most hardware stores, will make a good guide. Diamond blades for dry cutting do minimal damage to the edge of the cut. Dust Collectors that attach to grinders and saws are very useful to acid stain applicators.

If patterns are cut before staining, cut just before cleaning the surface prior to staining. Saw dust containing free lime can bind to the surface, causing distortion. If cut after staining, do so after the first coat of sealer has been applied.

Applying the Acid Stain

New concrete must cure for 28 days before starting work. Depending on the type of concrete, temperature, etc., stains can be applied in as little as 21 days.

When you apply the stain, consider the following:

  • Humidity and moisture content will affect the stain reaction. For color consistency, make sure that the moisture content of concrete is the same for all surfaces that are stained.
  • Staining, sealing, and covering the surface before and after other construction trades work on them will save on preparation and cleanup, giving you a better looking installation.

There are many ways to apply the stain, each method offering an alternative final appearance. Here are some application guidelines:

Sprayers are usually used to apply the stains, but they must be designed for acid and have no metal parts. Acid will quickly destroy metal parts and may affect the color of the stain. Use a spray tip with a circular pattern, spraying in a pattern that goes from left to right then from right to left, with someone using a large medium bristle brush to work the stain into the surface just behind the spray. It is important to scrub in the stain and not just push it around. An additional spray pass just behind the scrub removes brush marks. This method ensures good penetration and minimal marking from spraying or brushing.

EnduraColor Reactive Stain Extender can be used to dilute the stain to achieve lighter colors.

Acid stain applied with a brush will penetrate well, but be careful to minimize brush marks, which are usually regarded as undesirable effects. What is the method of applying stain, be sure to completely cover the surrounding areas to avoid accidental staining. Spillages can be difficult, and in some cases impossible to remove.

The growing interest in acid stained surfaces is in the direction of more subtle effects. Applicators often dilute the stain with EnduraColor Reactive Stain Extender to produce less intense effects. For example, some applicators often apply the stain with a working dilution of 80% (20% stain extender). In this case, the contractor can gradually build up the color to meet owner expectations. Second and third colors can also be added in the same way to create a multiple color overlay.

A portion of professional applicators find that the addition of a concrete overlay prior to staining is the best solution for concrete surfaces that show damage or have been mistreated during construction. Overlay materials of can be stamped, textured and/or stained to create a new range of decorative options. Overlays are highly resistant to cracking and wear. As with any stain, it is wise to create a sample in an inconspicuous area on the same surface to ensure compatibility between the overlay and the stain, and then obtain approval from the owner if necessary.

Cleaning and Neutralization

Once the Acid stain reaction is complete, a layer of acidic residue will remain on the surface. This residue should be completely neutralized using EnduraPrep Neutralizer. Failure to completely neutralize the surface will prevent the sealer from adhering to the surface. Use a white cloth after neutralization to check for residue. If, after wiping the surface with the white cloth, you have color on the cloth, then you will need to scrub and rinse the surface until you can no longer wipe colored residue from the surface. Furthermore, using a pH meter to test pH of the surface is a good way to ensure that conditions are ready for the next step in the implementation of clear sealer.

Sealer Application

External stained surfaces usually are sealed using acrylic sealers so that the moisture can escape from the slab. Solvent-based acrylic sealers generally work better than water-based for outdoor use. A good application usually consists of two thin layers of a clear acrylic sealer for best results. A wise addition to the acrylic sealer would consist of applying Endura Seal Floor Finish. This adds a sacrificial coat that adds shine giving you general wear and slip resistance. In high traffic areas you might consider using a high-performance sealer. High Performance Sealers (epoxy and urethane) are much harder than ordinary acrylics, but they are much more expensive also. For application instructions follow the directions on the label of the sealer you have chosen.

Owner Approval

Despite the limited color palette, acid stain finishes are growing in popularity. Today, homeowners are going out of their way to install the concrete, just so that they stain their surface. Acid Stained floors are very easy to clean and maintain. Popular applications of acid stains include concrete counter-tops, sinks and showers, plaster and stucco walls, both inside and outside. Acid stain will chemically react with any cementitious material.

In talking with customers about the possibility of acid staining concrete, have them to describe the look they have in mind and provide images that show different types of applications. Show color samples to help in the decision process.