Acid Stain for Concrete can be applied to new or old, smooth 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 concrete. Metallic salts in a water-based acid solution react with concrete to permanently color the surface. Siliceous aggregates such as gravel or sand do not react with the stain. Surfaces containing a higher cement content will react more than one with less cement giving more intense colors.

Since every surface is different, results may vary from surface to surface. We always recommend testing a small area to determine how the final result will appear. Please 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 in that you can see all the flaws or finish marks through it.

Factors that may affect final results include:

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

Acid stain finishes for concrete do not require a lot of equipment for application. The applicator typically uses a garden sprayer that is completely plastic. Some prefer a fine-bristled brush or a combination of both. All equipment that will be in contact with the acid stain, such as sprinklers, must be acid resistant. Brushes for applying or spreading the stain should be acid-resistant, as well as colorless bristles.

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

Surface Preparation

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

First you will need to pour some water on the surface in several places to see if the concrete will accept the water. If it is not absorbed by the concrete, it may have a sealer on its surface. If so, the surface will need to be stripped with EnduraPrep Coatings Stripper. This stripper is ideal for removing coatings such as paint or acrylic dyes and dyes. If there is no sealant on the surface, but it does not absorb water, then your surface is too dense. This is usually caused by over-troweling the surface when the concrete was poured. It is very important to condition these types of surfaces so that they accept the acid stain. Surface conditioning is often the key to success. If it does not accept the water, it will surely have to be conditioned. Sometimes very dense surfaces need to be conditioned twice. When the surface is properly conditioned, it should feel like 120 grit sandpaper. To use EnduraColor Reactive Concrete Stain, DO NOT USE hydrochloric acid on the surface as it will deprive the concrete of the minerals necessary to react with the acid stain. A properly conditioned surface can be easily achieved with EnduraPrep Concrete Surface Conditioner, a safe alternative to 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, paint drips, taping adhesive residue, putty, cement or any other surface contaminants. Anything left on the surface will affect the final result of the surface. EnduraPrep Cleaner & Degreaser is excellent for cleaning any surface contaminants. You can dilute 10 to 1 to clean your surface and use stronger for stubborn surface contaminants.

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

Decorative patterns with stencils can enhance 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 uniform in color as possible, cut out the lines and patterns after staining is complete. Stains penetrate differently around cuts and crevices. If you want there to be a color change in a pattern line, cut the line first to form a barrier against stain movement. If sawn joints are to be grouted, complete staining and sealing before grouting to help prevent stain from discoloring the grout.

The patterns are usually arranged in pencil or chalk. Mark only where you are going to cut. Also, do not use chalk that is difficult to remove. There are many tools available for cutting pattern lines in concrete. Most installers use hand grinders and saws with tables, mounted against a fence. A 1-1/2″ extruded aluminum “L” angle, available at most hardware stores, will be a good guide. Dry-cutting diamond blades do minimal damage to the cutting edge. Dust collectors that attach to grinders and saws are very useful for acid stain applicators.

If the patterns are cut before staining, cut them just before cleaning the surface before staining. Sawdust containing free lime can stick to the surface and cause distortion. If cutting after staining, do so after applying the first coat of sealer.

Apply the acid stain

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

When applying the stain, keep the following in mind:

• Humidity and moisture content will affect the reaction of the stain. To achieve color consistency, make sure the moisture content of the concrete is the same for all surfaces being stained.
• Staining, sealing, and surface coating before and after other construction work will save on preparation and cleanup, giving your facility a better appearance.

There are many ways to apply stain, and each method offers an alternate final look. Here are some application guidelines:

Sprayers are normally used to apply the stains, but they must be designed for acid and have no metal parts. The acid will quickly destroy metal parts and may affect the color of the stain. Use a circular pattern spray nozzle, spraying in a pattern going from left to right then right to left, with someone using a large medium bristle brush to apply the stain to the surface just behind the spray. It’s important to scrub the stain and not just push it away. An additional spray pass just behind the scrub removes brush marks. This method ensures good penetration and minimal spray or brush marks.

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

Brushed acid stain will penetrate well, but be careful to minimize brush marks, which are generally considered undesirable effects. What is the stain application method? Be sure to completely cover surrounding areas to prevent accidental staining. Spills 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 at an 80% working dilution (20% tint thinner). In this case, the contractor can gradually increase the color to meet the owner’s expectations. The second and third colors can also be added in the same way to create a multi-color overlay.

A portion of professional applicators find that adding a concrete coat prior to staining is the best solution for concrete surfaces that show damage or have been roughed up during construction. Overlay materials can be embossed, 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 advisable to create a sample in an inconspicuous area on the same surface to ensure compatibility between the coating and the stain, then obtain owner approval if necessary.

Cleaning and Neutralization

Once the acid staining reaction is complete, a layer of acid residue will remain on the surface. This residue must be completely neutralized with EnduraPrep Neutralizer. If the surface is not completely neutralized, the sealer will not adhere to the surface. Use a white cloth after neutralization to check for residue. If, after cleaning the surface with the white cloth, you have color on the cloth, you will need to scrub and rinse the surface until you can no longer clean any color residue from the surface. Also, using a pH meter to test the pH of the surface is a good way to ensure conditions are ready for the next step in clear sealant implementation.

sealant application

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

owner approval

Despite the limited color palette, acid stain finishes are gaining popularity. Today, homeowners go to great lengths to install concrete, only to have their surface stained. Acid stained floors are very easy to clean and maintain. Popular applications for acid stains include concrete countertops, sinks and showers, plaster and stucco walls, both inside and out. Acid stain will chemically react with any cementitious material.

When talking to clients about acid-staining concrete, ask them to describe the look they have in mind and provide pictures showing different types of applications. Show color swatches to help in the decision process.