The Acid Stain Looked Good at First… What Went Wrong?

An up close look of acid staining that did not react properly.
Acid stain dusting off the surface prior to sealing due to lack of penetration and reaction. Photos by Rick Gilmore
Question

I just stained a concrete floor in a new restaurant using acid stain. We used a power-trowel and the concrete had a very smooth, hard, dense finish. We mopped the floor with soap and water, did a clean-water rinse and let it dry overnight. Then, we came back the next morning, protected the walls and sprayed the stain using a pump-up sprayer. We let the stain sit all day and came back eight hours later. The restaurant owner loved the color and the way it looked.

We then washed the floor with soap and water and removed almost all of the color. The floor now looks spotty and ugly. Why did all the color come off, and what can I do to make it look like it did before I cleaned the floor?

Answer
There are a few different things going on that led to the floor looking spotty and unacceptable. The first red flag is that there is no mention of any sample being completed prior to the project start. I, along with most every stain manufacturer, strongly recommend sampling on all stain jobs, especially on commercial projects. troubleshooting acid stained concrete can help.

There are a few different things going on that led to the floor looking spotty and unacceptable. I do not see any mention of a sample or mock-up before the project start. This would be the first red flag. I, along with most every stain manufacturer, strongly recommend sampling on all stain jobs, especially on commercial projects.

And when I talk about samples, I mean a sample placed in an inconspicuous place on the actual floor that have the stained concrete on it. This includes all the steps that will from the actual project, including sealer and wax. These types of mock-up samples are frequently placed in a closet or bathroom, or in an area where furniture or other flooring will be installed. Too often I see samples created on boards or concrete chips that have no relevance to the concrete floor that will be stained. It will significantly change the look and color of the final product, which is why it is important to seal the mock-up or sample.

It is important to note that samples are not just about picking a color. You as the installer should be paying close attention during the sample process to make sure the stain is taking properly, the colors are developing and you don’t have any red-flag issues like fish-eyeing or orange-peel effect that indicate contamination in the concrete.

I have worked with some installers who take the time and walk the area, spraying the entire floor with water looking for contamination or trouble spots. Today’s concrete often contains fly ash and other pozzolans and chemical admixtures. While these can be good for concrete strength, durability, finish and cost, they can often retard the stain’s ability to penetrate or develop the proper color. A good rule of thumb is that if you have trouble getting plain water to absorb into concrete, chances are you will have trouble getting a stain to absorb. It is a lot easier and cheaper to discover a concrete issue by spraying water before you’ve started the job than after you have sprayed the floor with stain.

You mentioned in your question that the floor had a “very smooth, hard, dense finish.” This should have been an immediate warning sign that the concrete was probably going to require additional preparation. It always surprises me that many installers don’t think that floors require surface preparation when acid-stain is the chosen application. In some cases they don’t, but in many situations, especially on commercial flooring where the concrete is machine-finished, you do.

The surface preparation you described in your question — “We mopped the floor with soap and water, did a clean-water rinse and let it dry overnight” — falls a bit short, especially on a hard, dense floor. In many cases a floor with a machine finish requires sanding or a mild acid etch to break the surface tension of the concrete. If you do use acid to etch concrete prior to acid staining, I recommend using a diluted phosphoric acid detergent to treat the floor.

In this situation, the stain looked good before cleaning it because all the mineral salts (which are what react with the concrete to produce the color) are lying on the surface as residue. The concrete was both too dense and hard or there was some type of contamination was keeping the stain from penetrating and reacting.

A hard troweled floor after the acid stain was cleaned off.
A hard-troweled floor after the acid stain was cleaned off. The blotchy finish is the result of a lack of penetration and reaction on the part of the stain.

Always remember that there is a process by which all stains must go through to work properly. First, the surface must be porous enough for the stain to break the surface and get inside. The second is penetration, where the stain migrates into the surface paste of the concrete. The third is reaction and/or adhesion. This is the ability of the stain to react with cement-based material to develop its color. Missing or reducing any of these three steps slows down or even stops the process, resulting in poor stain color development. Whenever I troubleshoot a stain issue, I always look at porosity, penetration and adhesion/reaction. (Similar process can be noted for dyes and sealers, too.)

In regard to fixing this stained floor, I would start by cleaning off the residue from the first staining attempt. I would then prepare the surface by either sanding with a walk-behind and pad. You can also lightly etch with a diluted phosphoric acid detergent. Once that is complete, stain the floor, and consider using brooms to work the stain into the concrete. This promotes better penetration and reaction. The last step would be to properly neutralize and clean the floor to get it ready for the sealer.

I strongly recommend sampling this process to dial in the right steps. This will make sure you obtain the results you are looking for.


Acid stained problems after rubber mat was placed
Photo courtesy of Concrete Decor reader

Questions from Readers

Question

I had my floors screeded and acid stained 1 year ago. It looked great initially but now I’m noticing air pockets where it looks like the stain and sealer are separating from the concrete. Additionally, there are dark acid smelling wet spots underneath some newly placed floor mats. I have also noticed areas that have turned white. It looks like small bubbles or gases may still be coming off. Should I be concerned about fumes? What could be the cause of this? My concrete guy doesn’t seem to know.

Answer from Concrete Decor

Thanks for sharing your project concerns with Concrete Decor.

Regarding the acidic smell, was the concrete neutralized after the acid staining application? In other words, did your contractor wash or scrub it with a mixture of baking soda and water and then rinse thoroughly? If not, the concrete will maintain a high pH level until you do this. It can also be responsible for the sealer delamination you’re seeing.

We don’t recommend placing a nonbreathable mat on this type of surface. Not seeing the surrounding space, you may have moisture below the slab. When a rubber mat is laid onto your concrete surface, temperature changes can cause sweating (moisture hydration). The moisture will wick its way up through the concrete and lift your sealer off the concrete. Moisture is very powerful when it wants to go somewhere.

The coating applied to your concrete is also important to note. This can help with prescribing the proper remedy.

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