Stain Removal for Stone, Concrete, and Other Porous Surfaces
By Fred Hueston
Magic marker scrawlings, wine stains after a party, rust from a filing cabinet, oil spots from the car… stain removal for stone, tile, and concrete can be a major problem that stumps both customers and professionals. The following guide is designed to give you the knowledge and tools necessary to remove the most difficult stains.
The art of stain removal
Stone, tile, and concrete stain because they are porous, which allows fluids to enter the material and become imbedded below the surface. The size of the pores determines what will enter and become imbedded.
Some materials are more porous than others. Honed and textured surfaces are usually more porous than polished surfaces, so they generally absorb more staining material. The longer the stain remains, the deeper it penetrates, and the more permanent the stain becomes. Certain chemical reactions can also permanently set a stain. This is one reason to advise customers to remove a stain as soon as it occurs.
To remove a stain, we must reverse the staining process. In other words, we need something that is more porous than the stained material to literally suck the stain back out and into the more porous material. This porous material is called a poultice.
The art of stain removal for stone and other porous surfaces involves identifying the stain, choosing the proper chemicals and poultice material, and applying them correctly to remove the stain. This sounds simple enough, but there are many factors that determine if and how a stain can be removed. This article will take you through the process of stain removal step by step.
Is the stain a stain?
When asked to remove a stain from stone, tile, or concrete, you must first verify that the problem is, in fact, a stain. There are many problems that look like stains but are not.
Etching: Some highly polished concrete and tile and nearly all polished marble will become discolored and dull after coming in contact with acidic substances such as orange juice, lemons, soft drinks, household and commercial cleaners, etc. The dulling effect caused by acids is a condition known as etching, and it is especially common in marble countertops.
Etching is not a stain; the polished surface actually becomes damaged when it comes in contact with an acid, so removing an etch requires refinishing and repolishing. Though often confused with staining, etching cannot be fixed with stain removal techniques or chemicals. If the spot in question is dull, clouded, and whitish, it may be an etch. Feel the spot. If it is not as smooth as the surrounding surface, you can be sure it is etched; however, a mild etch may still feel smooth.
The simplest way to determine if a marble surface is mildly etched is to place some polishing powder on the etch, take a white pad with a little bit of water, and work the powder into a creamy slurry, rubbing the slurry across the spot for several minutes. If it is an etch, this process is likely to remove it or improve it. For deeper etches, this simple technique will not work very well, and you will need to re-hone the area before you can polish it.
Water spots: Another common problem associated with staining is the deposit of water spots and water rings left behind from a glass. These rings appear on marble tables and countertops when slightly acidic liquid runs down the sides of a glass and etches the water ring into the marble.
Chemicals in the liquid can also deposit minerals such as calcium on the stone, sometimes referred to as hard water spots. These mineral deposits are the same type that appear in an automatic dishwasher or on a glass shower door. These rings and spots are usually not stains and cannot be removed with stain-removing chemicals and poultices. Again, refinishing and re-polishing will probably be necessary.
Efflorescence: Efflorescence is a deposit of minerals that appears as a white powdery dust on the surface of stone or concrete. These minerals usually come from the setting bed or from the stone or concrete itself. When it becomes wet during installation or afterward, the water dissolves some of the minerals in the setting bed and carries them to the surface. When the water evaporates, the minerals are left behind in the form of a powder. If you wipe your hand across the surface and pick up a light powdery residue, you are dealing with efflorescence.
Stuns: Stun marks appear on certain marbles as white marks, but they cannot be felt if you run your finger across the mark. It seems as though it is below the surface. Stun marks are usually caused by an impact on the surface of the stone, such as someone dropping a heavy object or walking across the floor with high heels.
These marks occur from an explosion of the crystals within certain marbles, and they can be deep, extending all the way through the marble. Stun marks are very difficult to remove, but again, they are not considered stains.
Wet stone: When stone, tile, or concrete becomes wet, it tends to darken. This is especially true when newly installed because the setting bed is usually very wet, and the water migrates to the surface to escape and evaporate.
The drying process for newly installed or saturated materials can be lengthy depending on the temperature, humidity, and air flow. Certain granites can take months to dry and the moisture might appear blotchy like a stain, but it is important not to treat moisture as if it is a stain.
The best way to test a surface for moisture is to use a moisture meter. If you don’t have a meter, take a heat gun or hair dryer to the suspected wet area, and see if the area lightens. Caution: Do not apply too much heat, especially to granite, because it may cause the crystals in the stone to expand and spall or the stone to crack.
It is also possible to run into combinations of conditions, such as a stain and an etch. Wine is an example of this condition. The tannin in wine will stain marble, and the acid will etch it. In this case, it is necessary to first remove the stain and then refinish or repolish the etch.
Factors for stain removal
Once you determine that you are dealing with a stain, several factors impact how difficult the stain removal will be—or if it is even possible.
Type of stain: If you have ever tried to remove magic marker ink from a shirt or blouse, you know how hard that can be, while certain food stains, although they may look bad, are relatively easy to remove. The same is true of stone, tile, and concrete. Certain stains are very difficult to remove because of their chemical nature or because the cause of the stain is actually part of the stone itself such as iron deposits in marble.
Age of the stain: The older a stain the more difficult it is to remove. I received a call one afternoon from a homeowner who was upset with her remodeler. Apparently, the remodeler had used a black permanent marker on a white marble floor to mark where he was going to place a wall; however, in the end, the wall was constructed two inches back from the marked area, leaving a long black line parallel to the wall. When I asked the customer how long the mark had been there, she told me approximately 30 days. The remodeler told me he did not understand why the marker would not come off since he had first tested the marker by placing a mark on the floor and immediately wiping it off. What the remodeler failed to realize was the ink from the marker slowly penetrated into the stone. I knew right away that this was going to be a difficult task, but I was still successful at removing the marker. It took several applications of poultice, but it worked. Determining the age of the stain will give you a good indication of how difficult it will be to remove.
Size of the stain: The size of the stain also will influence how long it will take to remove. A small area stained with a few drops of oil is going to take a lot less time than an area that has had several gallons of oil spilled on it. It is important to find out how much of the staining substance has penetrated. I have had several instances where there was so much material spilled on the floor that it soaked all the way through and into the setting bed below. In these cases, it was nearly impossible to remove all of the stain.
Proper selection of poultice or cleaner: The improper selection of chemicals or poultice materials can worsen or permanently set a stain. For example, it is important to know what to use on iron stains because the wrong chemical can oxidize iron to rust and cause permanent staining. One chemical that rapidly oxidizes iron is common, household bleach. I have seen numerous instances where bleach was used in an attempt to remove a rust stain, only to make matters worse. The stain becomes darker and larger and is permanently set. Ask your customer if they have used any chemicals on the stain already to determine if the stain is removeable.
Generally, stains can be classified into two types:
- Organic stains are caused by those materials which are derived from living organisms. For example, most foods, drinks, and plants and some dyes are all considered organic stains.
- Inorganic stains are those materials which are not derived from living organisms. They are usually mineral-related such as copper and rust.
Why is the distinction between these stain types so important? Inorganic stains are mineral in nature, and so are natural stones, tile, and concrete. Iron is a compound found naturally in stone, some tile, and concrete and will oxidize and rust, causing the surface to turn yellow, brown, or red. This occurs frequently in white marble and some limestones. If the stains are caused by oxidization of iron, they may not come out, and knowing this from the start can help to save time and set expectations with your customer. Organic stains can also be difficult to remove for different reasons. For example, when oil is absorbed into a porous surface, it can quickly spread throughout the material even if the stain on the surface appears relatively small. To illustrate this point, place several drops of oil on a piece of light-colored marble or concrete; then turn it over after several days and you will see that the stain has spread below the surface. When it comes time to remove the oil stain, it’s important to understand that the true stain may be much larger than it appears. Of course, the quicker you can get to a stain, the less chance it will have to soak in and spread.
It cannot be emphasized enough how important it is to identify the stain. If the stain is unknown and you try to remove it, you could be using chemicals that may not work or may set the stain and make it permanent. Before you do anything else, ask what the stain is. Chances are the customer who called you to remove it knows what was spilled there. Knowing the identity of the stain is half the battle—then a proper chemical can be chosen to remove it.
If the stain is unknown, you will need to do a little detective work to try and figure out the most likely cause of the stain. The following steps comprise my investigative process for determining the type of stain:
- Look for color and consistency. If the stain has the same color and is spread entirely over the surface, it is likely caused by external staining materials such as old wax, crystallization fluids, iron in cleaning water, etc.
- Consider the location of the stain. Stains near the stove or refrigerator may have been caused by food or cooking oils.
- Look for a pattern in the placement of the stain. A splashed pattern indicates a liquid stain, while a smudge may indicate something solid was dropped. In Statuary white marble, iron might be noticed adjacent to veining, usually running alongside the length of the vein. If this is the case, it is a good indication that the iron is part of the stone and cannot be removed.
- If further analysis is needed, remove a section of stone to determine if the staining is on the surface only or through the entire stone. If the stain is all the way through the stone, then it will be difficult or impossible to remove. Examine the setting bed where you removed the section of material. If it is stained, remove a portion of the setting bed to see how deep the stain is or to determine if the stain is coming from somewhere within the setting bed. Often, rebar or stray nails and screws can cause iron staining.
What is a poultice?
We’ve established that porous surfaces stain because they are absorbent. In order to remove the stain, this process must be reversed and the stain sucked back out. A poultice is an even more absorbent material applied to a surface to draw out a stain; however, a penetrated stain is very difficult to reabsorb, so something must be used to first loosen the stain. This loosening is accomplished by adding a chemical to the poultice that is appropriate for the type of stain. Again, this is why stain identification is so important.
A poultice can be paper or gel, but the most common poultices used today are powders. A number of powders are very absorbent and are ideal for stain removal including:
- Clays and fuller’s earth,
- Chalk (whiting),
- Sepiolite (hydrous magnesium silicate),
- Diatomaceous earth,
- Methyl cellulose,
Clays and diatomaceous earth are usually the most effective poultices for stain removal. Do not use whiting or clays containing iron because any acidic chemicals used will react with the iron and may cause yellowing of certain stone surfaces. It is best to purchase poultice powder materials from a reputable supplier for this reason. Some typical paper poultices include cotton balls, paper towels, and gauze pads. Paper poultices can be quite effective on mild stains. They are easier to apply than powder poultices and are also easier to remove.
Gel poultices are usually thick chemical gels that are designed to be applied to a stain with the use of powders or papers. They work effectively with certain stains.
When purchasing poultice materials, ask if they contain stain removing chemicals or if they need chemicals added. Some powder and gel poultices contain chemicals, and all you need to do is add water. Never mix additional chemicals with a poultice that contains its own chemical formulation.
Steps for stain removal
Stain removal can be difficult, but the following procedure will make the job much easier:
Step 1: Identify the stain.
I continue to emphasize, knowing stain type is half the battle in stain removal. Ask questions and investigate.
Step 2: Clean the stained area.
Just because the stain looks like it is deep doesn’t mean that it can’t be cleaned with a good cleaner. Clean the area thoroughly using a cleaner with a neutral pH. A heavy-duty cleaner also may be used, but first try a mild, neutral cleaner. When attempting to remove any stain, always use the gentlest method first, then proceed to more aggressive chemicals and techniques. Cleaning will also remove any surface residue caused by the staining material, allowing for faster removal if a poultice is later needed.
Step 3: Remove coatings.
If the stone, tile, or concrete has been coated with wax, acrylics, urethane, or any other topical treatments, it is important to strip the coating before attempting to remove the stain. Most coatings will interfere with the effectiveness of chemicals used to remove the stains.
Step 4: Pre-wet.
Wet the stained area with distilled water to fill the pores with water. This isolates the stained area and prevents the chemicals used from drying too fast, allowing them to stay in contact with the stain longer.
Step 5: Prepare the poultice.
If a powder poultice is to be used, mix the powder with the stain-removing chemical you choose. Create a thick paste about the consistency of creamy peanut butter. You want the mixture to be wet, but not so wet that it runs.
If a paper poultice is used, soak paper in the chemical of choice before applying to the stain.
If a gel poultice is used, apply gel directly to the stain.
Step 6: Apply the poultice.
Apply the poultice to the stained area, overlapping several inches beyond the stain in each direction. This overlapping is important because the stain may be spread farther than it appears on the surface. The thicker the poultice is applied, the longer it will take to dry; generally about ¼ inch thick is sufficient for most stains. (See the step-by-step video on how to apply a poultice for stain removal below.)
Step 7: Cover the poultice.
Covering the poultice prevents it from drying out too quickly. It is necessary to keep the poultice wet as long as possible to allow the chemical to work on the stain and loosen it. Cover the poultice with plastic and tape down the edges with a tape that will not stain. Plastic food wrap and a low-contact masking tape work well.
To prevent the poultice from staying too wet, poke several small holes in the plastic.
Step 8: Remove the plastic cover.
After approximately 24 hours, remove the plastic from the poultice. If it is not yet dry, leave the poultice uncovered until it is. This is extremely important because a poultice works by drawing the stain out, and the drying process is what pulls the stain from the surface into the poultice.
Step 9: Remove the poultice.
After the poultice is thoroughly dry, scrape it off with a razor blade or putty knife. Be careful not to scratch the surface. Clean any residue of poultice from the stain with water and a neutral cleaner.
Step 10: Examine the area.
Carefully examine the stained area. If the stain is not completely removed, reapply the poultice. It may take several poultices to remove difficult stains. If after two applications of poultice you do not see any lightening of the stain, then your chemical choice may be incorrect, or the stain may not be removable.
Stain removal is not an exact science. The steps I have outlined will work in many cases, but some stains may be permanent and will almost never come out. Be careful not to waste too much time in applying poultices. If the type of stone or tile is still available and you can obtain a close match, it may be easier and cheaper to replace the stone or tile. In the case of concrete, it may be easier to remove and patch the stained area.
Once you have examined and identified a stain, be sure to discuss all the options and possible outcomes with your customer.
Fred Hueston is the owner of Stone Forensics, a leading consulting and training firm specializing in stone, tile, and other surface failures. He is the host of the weekly radio/podcast, the Stone and Tile Show. He is an author of over 35 books and hundreds of articles on stone and tile restoration, maintenance, fabrication, installation, and failures. Contact him at [email protected], and find his complete stain guide at www.surphaces.com/stain-management-app.
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February 21, 2023