We are often asked what method is the preferred best method for preparing the surface of a concrete garage floor, concrete warehouse floor, aircraft hangar, restaurant, office or show room floor for a resinous coating.
Many people are aware of the old saying, that paint is “only as good as the surface it is applied to”. This was true years ago and is still valid today. It makes little sense to buy an expensive resin and then apply that paint to a poorly prepared surface. Doing so will not likely yield good results.
So what are the common floor preparation methods? There are four (4) methods of concrete floor preparation that are common across the United States and Canada. These four methods are 1) Power Washing, 2) Acid Etching, 3) Shot Blasting and 4) Diamond Grinding.
Power washing, or pressure spraying, is exactly what it sounds like. An applicator rents or buys a pressure washer and then uses water under high pressure to clean the concrete surface. Sometimes this method might utilize a cleaner / degreaser chemical agent to help remove oil and other contaminants. These may be applied separately to the floor with a bucket, and often in a warm or hot water solution, and problem areas worked on with a wire or hard bristle brush to help remove the contaminants. Sometimes, depending on the model pressure sprayer being used, these cleaner / degreaser agents are added to a reservoir in the power washer itself and are mixed in at the pressure sprayer head. When these chemical agents are used, it is very important that the floor be thoroughly rinsed with fresh, clean water afterwards, to make sure there is no left over residue of these cleaner / degreaser agents left on the floor.
Power washing is an inexpensive floor preparation method. Equipment is readily available for purchase and rental in most market areas. The equipment can be purchased for a few hundred dollars in most cases and rented for well less than a hundred dollars a day. One downside to this method is that if too much pressure is used, or the head of the spraying unit is held too close to the concrete surface, the water can literally dig or chisel the surface, destroying the smoothness of the surface and causing a potential area of visual non-conformance that will draw the eye to it when the floor is coated. Another disadvantage to this method is that the floor will be very damp afterwards, and if using solvent based coatings, the floor will need to thoroughly dry before the coating is applied. In warm, dry climates, like the desert American southwest, this may take 1 to 3 days to dry out. In cooler, more humid areas, like the American Midwest or Florida (high humidity), it may take 5 or even more days for the floor to dry sufficiently for a solvent based coating to be applied.
In another article that we plan to post in the near future, many floor coating contractors use water based coatings, and while these coatings have their advantages and disadvantages, if a water based coating is used, this drying time delay can be avoided. This can become a driving factor in why some floor coating applicators prefer water based coatings, if they only have access to this less expensive water based floor preparation equipment, and then become motivated to avoid the delays in waiting for the concrete substrate to dry before being able to coat it.
Another commonly encountered method to prepare concrete surfaces for application of a floor coating, or paint, is acid etching. In this method, an acid, such as muriatic acid, a commonly used pool chemical and available in nearly every market, is diluted with water and applied to the concrete floor. The diluted acid and water mixture is allowed to sit on the floor for 30 to 60 minutes. During this time, the floor may “bubble” or “fizz” and appear to be cleaning the floor. What is really happening is the diluted acid is breaking down dirt and other contaminants, such as oil, rubber residue from tires, silicon and other chemicals such as armorall tire care products that people like to put on their tires to make them look new and shinny. Some people believe that this process also opens up the pores of the concrete and helps it accept and allow a resinous coating to more easily “wick” into or “wet” the floor when it is later applied.
After allowing the acid solution to sit on the floor for a period of time, usually less than an hour, an applicator then typically goes over the floor with an ammonia and water solution. Acid, as the name implies, has a pH level of less than 7, 7 being neutral like water. Ammonia is a base, or is alkaline, having a pH level greater than 7. The purpose of the ammonia water solution is to neutralize the acid water solution already applied and sitting on top of and which has soaked down into the concrete surface. For optimal results, paint should be applied to a pH neutral substrate. If a coating is applied to a surface that is acidic (pH less than 7) or base (pH greater than 7), the acid or base will over time eat into and destroy the paint, cause adhesion failures, and or what appears to be random bubbling.
When considering acid etching as a floor preparation method, keep in mind that concrete is a porous substance. It has lots of air pockets and tiny holes throughout its depth. It is in essence a giant sponge, just one that we cannot easily pick up and squeeze. And herein lies the main disadvantage of acid etching. When the acid water solution is applied to the floor, think of the structure of a sponge and mentally picture the acid water solution absorbing down into this sponge. When the ammonia water solution is applied over the top of this later on, to neutralize the acid solution, some of this acid sitting at the lower depths of the concrete will not be mixed with the ammonia solution and hence will not be neutralized. Some of the better floor coating contractors realize this and take extra steps to minimize this problem. They may flush the floor with water before applying the ammonia solution, to help reduce the amount of acid that needs to be neutralized. This water runoff, being acidic, can present other problems, such as discoloring and scoring or “burning” concrete downstream where the runoff goes. This acidic runoff can also damage or kill plants, if they are in the downstream path, such as alongside a driveway. Some cities and other jurisdictions have prohibited this method because of the issue with acidic runoff and downstream waste water contamination considerations. Even if this intermediate flushing (before the ammonia solution is applied) is not used, a flooring contactor using this floor preparation method will want to very thoroughly flush the floor with copious amounts of water afterwards, as it is not desirable to have either acid or base residue on the floor, or down in the concrete, as anything other than a pH of 7 can become an impediment to good long term adhesion.
Notice how some parts of the floor are good and others are completely destroyed? This is a few years after a homeowner acid etched and epoxied their own garage floor. These issues come about because of a failure to neutralize the acid in the floor and years later it eats through the coating causing bubbling and adhesion issues to the point where large sections will peel off the floor.
Most, if not all, of the DIY big box hardware stores selling do it yourself garage floor coatings provide the chemicals or recommend going the acid etching preparation route. And many people who have applied these DIY garage floors and who initially think they have saved money over the cost of having a flooring contactor who is in the business of doing floor coatings, are later disappointed, usually a year or two after application, to discover their beautiful floor now sports bubbling in random patterns across the floor. We have talked to many of these people over the years. They are confused as to why their floor continues to look good in some areas, but has these bubbles or adhesion / delamination issues randomly appearing in other areas (see photo above). Our best experience and knowledge tells us this random bubbling up of their floors is from the left over, non-neutralized acid. Remember, concrete is a sponge. This means it will wick in and absorb moisture from the ground around and surrounding the slab. As the rain and snow comes (and melts), this moisture can be pulled into the concrete slab. As temperatures increase, the moisture wants to rise up thru the concrete and migrate towards the surface. The non-neutralized acid still trapped in the pores of the concrete, below the now dried floor coating that was applied 1, 2 or more years prior, is pushed up by this moisture and eventually contacts the bottom layers of the dried coating. Over time, this non-neutralized acid eats these sections of the dried floor coating in those areas where the pores had the acid trapped, resulting in what now appears to be random bubbling patterns. This is a major disadvantage of acid etching.
In our experience, only the very best and most experienced floor coating contractors understand this reality and take the necessary extra efforts to try to overcome and minimize this issue. This is why we tend to not recommend this type of floor preparation. Another disadvantage of this coating is the odor of ammonia. Ammonia is a very, very strong, and for most people, unpleasant odor. Suitable respirators should be used. It is also worth noting that when the contractor is finished with this floor preparation method, the floor will be damp or wet. Like the comments in the section above on power washing, if using a solvent based coating, the floor will then need to be allowed sufficient time to thoroughly dry. Contractors that tend to use this floor coating method, for this reason and to avoid the drying time, tend to use water based coatings, so that the first coat of resin can be applied shortly after flushing the floor with water.
Shot blasting is a method that was used commonly about 10 or more years ago and is quickly becoming less commonly used. Shot blasting is exactly like it sounds. A machine is loaded up with small steel balls. These balls are shot at high speed under pressure towards the concrete. When the balls hit the concrete, they literally blast loose a layer of the top surface into pieces that crumble away and can be swept up. These machines leave a fairly rough profile. If using a thicker resin like epoxy, this rougher profile is acceptable and the thicker epoxy resins will self level when drying and result in a smooth surface. However, if using one of the newer resin technologies, such as a polyaspartic polyurea, which are designed to be applied in thinner layers, this rougher surface left by the shot blasting machine may not be sufficiently smoothed over by the thinner applied polyureas, resulting in a floor finish that is not smooth and therefore may not be desirable. As these newer resin technologies have become more widespread, as they offer many desirable and improved dried film performance characteristics over resins like epoxy, shot blaster machines are being used less and less in the field.
Scarifer machines are similar to shot blasters. Instead of using steel balls to shoot into the top surface of the concrete, these machines typically have multiple blades, arranged vertically, on a rotating horizontally disposed shaft. The blades are staggered and designed in such a way, that they provide a very aggressive chipping action to the surface of the concrete. These machines often produce what is famously referred to as “corn rows”, valleys and peaks in the floor surface after the machine passes over. The valleys are where the blades dug into the concrete and the peaks are the spaces in between the blade spacing. A contractor typically has to make multiple passes with this type of machine to knock down the “corn rows”, and often has to use another floor preparation machine to smooth out and tone down the roughness, or profile of the floor, as the scarifer, when used alone, typically leaves a floor that is too rough and uneven to coat. These machines are most useful, where trying to remove a coating that was previously applied is very hard and difficult to remove. Even this advantage has been largely negated in recent years with improvement in the shapes of and designs of diamond cutting blades used in concrete diamond grinding machines.
One of the newer and and in our opinion the current best practice for concrete floor preparation, is diamond grinding. Diamond grinding uses specially designed machines that accept interchangeable and replaceable diamond embedded grinding tool bits into the rotating head portion of the machine that comes into contact with the concrete floor. These machines offer dry, non-aqueous, non-chemical means to remove a thin layer of the concrete and the dirt and contaminants in that layer. Many of these machines interface with accompanying concrete vacuums that suck up the concrete dust that is created in this process. Use of these machines typically result in a concrete surface that looks like it is brand new and which is clean and free of contaminants. Dirt, oil stains and silicones from car care products, are all typically ground out of the floor and the resulting dust swept up by the vacuum. A word of caution here. If dealing with extensive oil or other chemical stains, it may still be necessary to employ additional methods, such as a cleaner degreaser to loosen up and help raise the oil from the floor, before grinding. Concrete diamond grinding machines seem to come in all sizes and shapes these days.
Diamond grinders are manufactured by an increasing number of companies and available in many different sizes, shapes and weights. Having a machine of sufficient weight for the job is an important consideration. Also having a machine with a motor that is able to apply sufficient power to the diamond grinding bits is also a factor to consider. Many machines are electric and require more than the commonly available 110 volt power found in practically every modern home. For this reason, many contractors that use these machines also have and bring to the jobsite their own generators for producing the higher voltage and wattage needed to run these machines. Some of these machines are even gas powered, not electric, and some newer ones are even propane powered now.
We think the better machines have counter-rotating heads. The outer head will typically spin in one direction. Attached to this head with be another head with 2, 3 or more smaller heads that rotate in an opposite direction. Attached to these smaller heads will be several individual diamond bits. Diamond bits are available in different “grits”, like sandpaper for example, allowing a user to adjust the aggressiveness of the grinding based on the jobsite concrete. For softer concrete, less aggressive bits are used. For harder concrete, more aggressive bits are usually employed. Diamond bits also are available in different shapes which provide different cutting performance. The smaller, less complicated diamond grinding machines can cost a few thousand dollars. The larger, more complicated machines can cost well above $10,000 each.
When using diamond grinding machines, there is no additional consideration or potential delays in waiting for a floor to sufficiently dry out before applying a solvent based coating to your floor. The biggest advantage of using diamond grinders is that this method likely gives you the best opportunity for removing dirt, contaminants and other barriers to adhesion. But it does come at a cost and is likely the more expensive of the options. This is a function of the cost of the machines and the replaceable, wear diamond bits that have to be replaced regularly. If your floor coating contactor does offer this type of floor preparation, then we believe this is a good sign that they are likely knowledgeable and committed to the profession, including the commitment to have on hand the best tools to do the best floor prep.
Conclusion of Concrete Surface Preparation Methods
So, what is the best concrete flooring prep method? The answer is it depends on your job and your budget. A solid and experienced floor coating contractor should have at their disposal several means that they are experienced in using, and should be able to offer you more than one method and be able to discuss the pros and cons as to why they are recommending the method they have. Probably the most commonly used floor preparation method is acid etching. Be aware that this is almost certainly the least expensive method and does enable many newcomers to enter into the business of floor coating. If dealing with someone who is recommending this method, we recommend you take the time to learn how long they have been in business, whether they employ other methods of floor preparation and why they are selecting or recommending this method for your job. There can be valid reasons why acid etching may be the best method for a given job and or your budget. However, make this decision with purpose and knowledge. What is not a valid reason is that this is the least expensive method and they don’t have or cannot afford diamond grinding machines.
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