Sealing Concrete Floors: The Complete How to Guide

Cause of dull spots in sealed floor

Question: Four years ago I covered my basement floor with a patterned overlay (I am in the decorative concrete business). I applied a two-part polyurethane sealer to it. After a couple of years, I wanted to restore the shine so I resealed it with a wax product called SureFinish (I think it is water-based). Over the course of a few months, the floor looked more and more dull in spots, especially where walked on with wet shoes. I stripped the floor with a mild muriatic acid cleaner and applied an acrylic solvent-based sealer, thinking that I needed the solvent to restore the color. The floor looked great at first, but after about a month it began to dull again in spots. When I wash the floor it looks great, but the dull spots show up when it dries, and over time I'm getting more and more spots. Do you know why this is happening? Is it perhaps because the original sealer was a polyurethane that the others sealers are not bonding to, and with washing and water exposure, they are coming off? How do I now reseal over the original sealer?

Answer: I think you are on the right track in terms of the other sealers not bonding to the original polyurethane sealer. Now you have multiple layers of different sealers, all of which seem to be wearing. Applying a water-based wax and then a solvent on top was also not a good idea.

What is probably happening is diffusion. Where a sealer coat starts to come off, light no longer travels through it but instead diffuses, resulting in dull or white and hazy spots. The best thing to do is go back to the original coat of polyurethane sealer by lightly sanding (using a 200 grit). Then apply very light coats (800 to 1,000 square feet per gallon) of the wax. Test this remedy in a small area first to make sure it is working.

Removing bubbles and roller marks in a sealer

Question: I recently applied a two-component, high-solids aliphatic polyurethane sealer to a stained floor in a spa. First, I applied the primer coat with a roller and then applied the polyurethane, which has a longer pot life, with an airless sprayer. While applying the primer on the first half of the floor, I could see roller marks and bubbles and the primer started to string up on me. I mixed a new batch and proceeded to finish rolling the rest of the floor but at a much faster pace, and I seemed to have no problem. But now roller marks are visible throughout the entire floor, not to mention the bubbles.

To remove the bubbles, I was told by the sealer manufacturer to go over the floor with a floor buffer or 150-grit sandpaper and then seal again. But will buffing be aggressive enough to take out the marks? Also, should I apply the primer using a sprayer instead of a roller, and should I mix smaller amounts of the sealer? I think I may have rolled on the primer too fast producing the roller marks. What do you recommend to fix the issue?

Answer: Sanding will take surface bubbles out, but buffing with a floor machine will not be aggressive enough. If the bubbles go deeper than the surface, you may need to strip and start over. My suggestion is to try sanding out the marks and then reapply your topcoat. If this does not work, strip down to a point were no roller marks or bubbles appear and start over. Do a test area before doing the entire floor.

I am not a fan of rolling in general, especially heavier bodied materials like epoxies or polyurethanes. All rollers produce lines, with more material coming off the ends of the roller as it moves across the surface. Depending on the material used, application temperature and pot life, the roller lines may or may not even out. The speed of rolling, the amount of material on the roller and the weight pushing down on the roller also play a role in leaving roller marks. Picture a car tire that has mud on it. If you drive slowly, the mud comes off the tire evenly. If you speed up, the mud flies all over and leaves more prominent marks on the outside edges of the tire. Heavy-bodied materials are always best applied in small batches and with a push-pull applicator such as a lamb's wool or micro-fiber pad or a T-bar.

Blotchy spots in a glossy sealed floor

Question: I recently applied two coats of an acrylic sealer to a stained floor. After the sealer dried, the floor appeared blotchy, with some areas glossy and others not. What causes this, and should I apply more sealer to remedy the situation?

Answer: If you were to look at a concrete floor under a microscope, you would see what looks like an endless range of plateaus and valleys. The number and depth of these plateaus and valleys is directly proportional to the finish of the concrete. A smooth finish has fewer of them while a rough finish looks like the Grand Canyon. When sealer is applied, it fills the valleys, but you may need to apply more than one coat depending on the finish of the floor. Typically when we see a floor that has blotchy areas of gloss, this means that some areas do not have sufficient sealer to fill the valleys. Applying additional light coats of sealer usually resolves this issue. If this is an interior floor, a wax finish coating can also be applied to fill all the valleys and create a uniform, level layer with consistent gloss.

Sealed surface is hazy white and blotchy

Question: On a project in Florida in mid-September, I applied a microtoppping on a Monday, acid stained the next day, and then neutralized and cleaned the stained surface the following morning. After letting the floor dry in 95 F heat for 2 ½ hours, I applied the sealer as directed. The sealer never became clear, and the entire sealed surface is hazy, white, and blotchy. What went wrong?

Answer: This is a common problem, more often found with water-based sealers. There is always a trigger mechanism for sealers becoming cloudy or white, and we need to timeline your project to find what it is.

You allowed approximately 24 hours of cure time for the overlay, which is standard and acceptable. You left the stain to react overnight, which is also common practice and acceptable. The problem lies in the short 2 ½ hours of dry time after you neutralized and cleaned the stained surface. Even with the high air temperature, the humidity was not taken into consideration. Most sealers need 12 to 24 hours of dry time before application, especially when a large amount of water is used in the cleaning process. So in this case, moisture was trapped under the sealer, causing the white, blotchy haze.

Depending on the type of water-based sealer you used, a thin application of solvent might loosen the sealer enough to allow the moisture to escape. Otherwise, you will need to strip the sealer, clean the surface and allow it to fully dry, and then reseal.

Four key factors to always be aware of when sealing are moisture, temperature, cleanliness, and application method. Also, read the label, because in this case it clearly stated minimum dry times of 12 to 24 hours before application. More information on sealing stained concrete is available in Bob Harris' Guide to Stained Interior Concrete Floors, as well as from most reputable stain and sealer manufacturers.

Correcting uneven gloss on a sealed floor

Question: I applied two thin coats of a high-gloss sealer to a stained concrete floor, following the manufacturer's instructions, but some areas of the floor look dull instead of shiny. What went wrong, and how do I fix the problem to achieve a uniform shine?

Answer: It's not uncommon for an uneven gloss to occur after two thin coats of sealer have been applied to a stained floor. The cause is different levels of porosity in the concrete surface. In some areas, the concrete is more porous and most or all of the sealer was absorbed into the substrate. These areas look dull because very little sealer remains on the surface to reflect light and produce gloss. In other areas, the sealer has saturated the surface of the concrete while leaving the desired film on the surface. This film is the protective barrier and reflects light to produce the level of gloss you want.

To eliminate the dull spots and produce a nice, uniform gloss, apply an additional coat of sealer followed by two or three coats of a floor wax or finish. These finish coatings are low cost, go down very thinly and are easy to maintain. They can be applied with a micro-fiber mop, and they dry in a matter of minutes. See this video on using sacrificial waxes for concrete floors.

Sealers perform differently, why?

Question: On a recent stained concrete project, some areas of the floor were sealed with a two-part polyurethane sealer, while other areas of the same floor where sealed with a solvent-based acrylic sealer. Why did the two-part polyurethane peel off, while the one-part solvent-based acrylic is still down and performing great? The surface has a hard-troweled machine finish, and no additional surface prep was completed prior to staining or sealing.

Answer: This is a case of chemical versus mechanical bonding and solids content. Two-part polyurethane sealers form only a mechanical bond with concrete. This is why profiling the surface – creating more surface roughness – is always called for when working with two-part polyurethane sealers.

The same reason two-part polyurethane sealers don't chemically bond to concrete is also what makes them such good sealers. They don't chemically stick to anything, and nothing chemically sticks to them, including oil, gas, solvents, water, dirt, and graffiti.

The other major factor is solids content. Most two-part polyurethane sealers are high in solids, above 55%. The higher solids content makes them less able to penetrate the concrete surface, thus requiring additional surface profiling to promote adhesion.

Conversely, one-part acrylics bond both mechanically and chemically with concrete and are lower in solids – typically 20% to 30%. They tend to wet out faster, even on hard-troweled surfaces. As illustrated by the case in question, an acrylic sealer may chemically bond to a hard-troweled surface, but the surface won't have sufficient profile, or roughness, to mechanically bond with a high-solids polyurethane sealer.

Most manufacturers of high-solids sealers require a well-defined profile before application of their products. The use of a primer or diluting the first coat of sealer are also common methods for promoting sealer adhesion on denser surfaces. Before applying a high-solids sealer, be sure to consult with the manufacturer and perform an on-site adhesion test. Find out more about two-part and one-part sealer technology and application methods from Surtec Inc.

Preventing moisture problems in concrete sealers

Question: I am doing a stained concrete project using an acetone-based stain. I plan on sealing the stained concrete with a water-based urethane. This is a large 8,000-square-foot floor in the cafeteria of a local manufacturer, and it receives heavy traffic. I just completed a moisture test (using a calcium-chloride test). I did three tests, with results that ranged from 6.13 to 6.58 pounds. But I understand the recommendation is no more than 4 pounds. This floor is on grade, and the concrete is over two months old with a vapor barrier under the slab. The interior humidity has been high (around 70%) due, I think, to rainy weather and wet drywall mud.

Here is my question: Because of the high moisture readings, should I do this job? Is 4 pounds a strict guide, or are there some tolerances? For example, is 6.58 pounds acceptable but something higher, like 12 pounds, too high? This is the first time I have done a test like this.

Answer: According to the ASTM standard for conducting a calcium-chloride test (ASTM F 1869), a reading above 4 pounds—the amount of moisture flow in pounds per 1,000 square feet over a 24-hour period—is considered a no-go for sealing. This is especially true for nonbreathable coatings, like the polyurethane you plan to use. That being said, you have a slab that should eventually drop below 4 pounds because of the vapor barrier.

You hit the nail on the head in regard to humidity. This is a very real environmental factor often missed when contractors prepare to seal, especially interior floors. Concrete is a sponge, and if conditions are right it will absorb and hold moisture, both in liquid and gas forms. The process of osmosis now comes into play—the movement of a liquid or gas from an area of high concentration to low concentration until balance is achieved. If the air above your floor has a high concentration of water vapor, or humidity, then the concrete will absorb moisture until it contains water equal to what is in the air. The practice of closing doors and windows to "dry-in" a room or building can actually trap moisture and create a steam room of sorts, making the problem worse or creating a problem where one didn't exist. Drywall mud, paint and rainy or hot, humid weather can cause high moisture contents indoors, which will cause concrete to absorb moisture. This moisture will stay in the concrete until the moisture content of the air above the slab abates. Whether the humidity is manmade or natural, it can cause short-term high moisture readings, especially in the spring and fall, when days are warm and nights are cool.

Wait for a dry spell, open as many windows and doors as possible, use a fan to move air across the floor, and take another test after 24 to 36 hours of drying. If the HVAC system in the building is operational, run the air conditioner to help remove moisture. When the moisture readings drop low enough for you to seal the concrete (at or below 4 pounds), apply the sealer in the afternoon or evening, when humidity is lowest.