Prepping Concrete for Stain: The Key to Lasting Results

The term “Stain” in the concrete industry describes a liquid coloring material used to impart color to existing, hardened concrete.

But what is stain? Stain can be a colorant in solution, suspension, or in a reactive form. A dye colorant in solution, like wood stain, will penetrate the substrate and not lay on the surface. The liquid stain color will be the same as the applied material. After the application, the carrier will begin to evaporate, leaving the colorant behind. Once the carrier is gone, the substrate will take on the intended color. This type of stain will be translucent. The substrate will show through the color.

A stain that is in suspension has pigment, binder, and carrier. The color of the liquid is the same as it will appear once applied. The carrier can be a solvent or water. A low-cost binder might be an acrylic. Once the carrier leaves the film, the binder cross-links and then “glues” the pigment to the substrate. This is a topical, film-forming coloring method that will be an opaque, solid appearance.

Acid stains are a chemical reactive coloring method. The color of the liquid stain is not the final color once it reacts with the concrete. Metallic salts within the material react with calcium hydroxide in the concrete producing color. Acid stain is not predictable. Every concrete slab will have a different appearance. The chemical reaction on one slab will not be the same as the next. Those can be the reasons why someone would be interested in using it. The final appearance will be unique with a level of variation.

The surface preparation for each one of these materials is important to obtain success. But what dictates the level of surface preparation needed?

Assessing Your Concrete Substrate

When selecting a stain, the evaluation of the substrate is critical. Not all stain types work on all substrates. That is why it is important to evaluate what you will be working with and have a good understanding of what the client’s expectations are.

There are times when the client hands you an image taken from the internet and says, “This is what I want”. Only one stain-type look is repeatable. That is the topical pigmented stain. The substrate has minimal impact on the color produced or the opacity it provides. The only thing that could influence the final appearance of a pigmented stain is the texture of the substrate.

With transparent stains, color variation of the concrete will influence the result, especially with chemically reactive stains. The level of reactivity will also enhance or diminish the color intensity.

Surface texture not only encompasses how smooth the surface is, but if there are cracks, joints, or spalls, they will also have their own texture. All stains build little to no film. They will not fill imperfections. That happens during the surface preparation step.

If the slab is new or with no imperfections that require repair, all three types of stain can be used but that doesn’t mean you can reproduce the same look as you see in an internet image. When it comes to translucent stains, concrete color, cement paste reactivity, method of application, and the age of the concrete will have an impact on the final appearance.

During the evaluation before applying the stain, everything that could affect color development needs to be documented and adjusted for. A good understanding of what you are working with will help ensure a successful project.

Essential Prep Steps

Apply all stains to a clean, dry surface. Remove all bond-inhibiting materials during the surface prep. This is done with the use of a concrete grinder fitted with diamond abrasive tooling in different grits. Niagara Machine has an array of equipment to choose from. Grinders, tooling, and air-handling equipment can be bought through them. If renting fits your needs better, Niagara Machine’s rental partner, Sunbelt Rentals, can be a huge resource.

When selecting the grinder tooling, the weight of the grinder, the hardness of the concrete, the size and configuration of the area, and the protective sealer used to protect the colorant are all important. It is much more than grabbing a grinder and starting the grinding process.

Grinders come in varied sizes, weights, and power requirements. When evaluating the substrate, decide what will work in the area you have. The weight can be associated with size but some 20” grinders have more head pressure than some 25” models. Grinders can be powered by electricity or propane. The electrical requirements can range from 110v to 480v 3 phase. Propane needs no electrical power but if working in a small space, adequate ventilation is necessary.

The tooling you will begin with depends on the concrete slab. Are you removing a coating or carpet adhesive, is it a new concrete floor or is it old? The coarseness of a diamond is dictated by what you currently have and how aggressive you need to be.

The last tool to use normally depends on the protective sealer. Some sealers need a “tooth” to obtain a full bond. An acrylic will need less of a profile than an epoxy. Figure out what your sealer requires.

Chemical reactive acid stains are a different animal. They need to absorb to react. They develop more character when the concrete has variation. If it is applied on a ground surface, the grinder removes the imperfections (variations) minimizing color variation. If the slab is hard-troweled, it needs to be opened to provide variation. That’s when a grinder needs to be used. Whichever preparation method is used, it needs to be mechanical. Using an acid to etch the floor will deplete the calcium hydroxide in the concrete which the acid stain reacts with. Your color will not develop as intended.

No matter which stain is used, always install a small mockup to obtain approval from the owner before you start. The mockup needs to be on site, but not in the same area you will be staining. Select an area that will be covered with another flooring material such as carpet or tile. This way you can hide it once finished.

Avoiding Common Staining Mistakes

The best way to minimize mistakes is to do your due diligence during the evaluation process and to install a mockup to obtain approval. Too many times contractors just jump into a project without a plan. The plan needs to have alternative avenues in case something pops up unexpectedly.

Before you arrive, walk through the project with the staff offsite. Make sure you have everything you need. Leaving the site to go buy something is money not well spent. Make sure you have the correct tooling and application tools. Will you need an air scrubber to capture airborne dust? How about power, make sure there is enough to run the equipment needed to perform the work. Have you read the technical data for the materials being used? If you have any questions, most manufacturers have a technical department to reach out to. You cannot plan enough.

All surface prep steps need to be completed to reduce the possibility of an issue. Trying to use inferior materials because of cost, rushing the mechanical prep process, or not considering high level moisture values could lead to an unacceptable result.

Conclusion

During the evaluation process, everything needs to be analyzed. The substrate conditions, moisture levels, concrete hardness, electrical power availability, and area accessibility need to be understood. To stain the concrete floor properly all aspects of the project need to be planned out ahead of time.

All repairs need to blend into the color and design. It is highly recommended to include all issues within the mockup to set the expectations with the owner. No mockup should be perfect. If perfect, the owner will expect perfection.

By following the manufacturer's printed installation instructions, the chance of a successful project will be elevated.

Working will your Niagara Machine field representative, you will gain more insight into the process, much more than just by reading the literature. These field reps have been involved with all types of stain from the evaluation process through the choice of the sealer.