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Grinding a garage floor versus acid etching is a topic of much discussion when it comes to preparing concrete for epoxy or other garage floor coatings. Also known as profiling, concrete surface prep sometimes comes as a surprise to people when they find out that you just can’t paint or coat over the concrete and expect it to adhere properly.
Typical questions asked are “Why do I need to grind or etch my concrete?”, “Which method is better?” or “Why do I have to etch new concrete?” With these questions in mind, let’s look at why concrete has to be prepared properly for a coating and which method may be best for you to achieve the results that you need.
Why Do I Need to Etch or Grind my Concrete?
Unlike applying paint, garage floor coatings need to penetrate and bond mechanically to concrete in order to adhere properly.
The problem, however, is that concrete for a garage is almost always finished to provide a smooth and even surface. This finishing process reduces the porosity of the concrete. As a result, the thick and viscous nature of coatings cannot penetrate and soak into the surface very well.
Profiling the concrete exposes the pores so that the epoxy coating can penetrate into the surface much better in order to get the best bond. It also creates a rougher surface for the coating to mechanically adhere to. This is typically done by either grinding the surface or by acid etching in order to expose these pores. Poor profiling/concrete surface prep, or lack thereof, is the number one reason why floor coatings fail and peel up.
Contrary to what some DIY installers assume, newly poured concrete or concrete from a new home still needs the proper concrete surface prep. Though it is fresh and clean without any stains, the surface needs to be roughed up and the pores exposed in order for the epoxy to penetrate and bond to it.
Which is best, grinding or acid etching?
When it comes to preparing concrete for a garage floor coating, the best method is to grind the garage floor.
The reason for this is that the profile of the surface can be controlled to provide the desired outcome. It provides a rougher and more porous surface that is considered ideal. It will also remove excess laitance that acid etching doesn’t always remove.
Concrete laitance is a very thin and weak layer of concrete at the surface which can be a result of the finishing process.
If a garage floor coating attaches to this weak layer, it can easily chip from impacts or eventually peel up, exposing a fine layer of concrete on the underside of the coating.
In addition, if you have any garage floor repairs that need to be made first, grinding will smooth out those repairs so that they don’t telegraph through the coating.
Professional garage floor coating contractors who warranty their work will always grind the concrete knowing that their floors will not fail due to insufficient concrete prep.
The problem with grinding your garage floor, however, is that it’s not always feasible for the average person to do so. There is the question of where to rent the equipment, how to control the dust, gouging of the surface due to inexperience, and finally cost. Sometimes it’s just not in the budget.
This is why acid etching is still a popular practice. If done properly, acid etching can provide a satisfactory surface for an epoxy coating to adhere to. It is fairly easy to do as long as you are safe.
It provides a better alternative to grinding for many and can be downright cheap in terms of cost.
The problem with etching is that the uniformity cannot be controlled like it can be with grinding. Because the density and finishing process of a concrete surface can vary, so can the effectiveness of the etching. This is why it is important to test multiple areas of the surface for porosity and texture once you have etched.
A simple method for testing is the water drop test. If the concrete immediately turns dark and absorbs the water, then that area is good. If it doesn’t, then you will need to etch those areas again. If you don’t test the concrete after etching, you run the risk of having areas of your coating that can fail.
The before and after images below are a great example of the differences between acid etching and grinding. This was an excellent demonstration that Epoxy Floor Co out of Australia did to show their customers.
The difference between the two prep methods is obvious. While the acid etched surface lightened the concrete slightly to expose the pores, the grinding removed the top layer of concrete, exposing the lighter color underneath and providing a rougher surface.
Note: If you have a sealer or coating on the concrete, acid etching will not work. You will have to grind the floor to remove it.
Additional Tips
If you are going to hire a floor coating contractor to install an epoxy coating in your garage, make sure that they grind the concrete. Any contractor that wants to acid etch is just cutting corners to reduce costs and should be suspect of providing a good quality floor.
If this is a DIY installation and you are fairly confident with your abilities, you can rent or purchase the proper grinding equipment necessary for the job. In most cases you should be able to grind a two or three car garage in less than a day. There is also the option of hiring someone to do it for you.
If grinding is not in the budget or you are not comfortable with doing it, then acid etching is your answer. If you are uncomfortable with using muriatic acid, there are a few commercially available products other than muriatic acid that can etch your floor safely as well.
Remember, acid etching will not remove dirt and oil from your concrete floor. It still needs to be cleaned first. Etching works by reacting with the free lime in concrete. If the concrete is coated in oil or grime, the acid will not be very effective.
Whether grinding or acid etching, if not done properly, your coating is destined for failure. It is the most important part of applying a garage floor coating and crucial that you do it correctly.
There is nothing worse than spending the time to apply a beautiful epoxy floor coating only to have it fail due to the improper profiling of your concrete. If you take the time to do it correctly, you will be rewarded with a coating that will adhere like it should.
Note: Shot blasting is another form of mechanically profiling your concrete that is widely acceptable to grinding and is used by some professional installers. Due to the cost of renting these machines and the experience required to shot blast effectively, we did not discuss this option here.
I was wondering about your thoughts on a bit of a change we have implemented. I have always been a fan of grinding over etching. Mostly because it is always effective when done right.
With the rise of thin to medium build coatings, especially high-end Polyurea coatings, we have been noting that
(a) The rental equipment for grinding is often not maintained. While that is hit or miss, it is becoming more and more of a concern.
(b) More importantly for DIY products, thin or medium build, all other things being equal, we are finding that less experienced installers are less prone to creating a profile, or even damage, that will require additional coats and or mitigation to correct.
So long story short, in this business we used to say you always want 100% solids. Then much of the industry realized that a small amount of solvents in the hardener makes for a much easier to install floor — even though you sacrifice a slight amount of Mil thickness. That represented a complete paradigm shift for many. Do we need to start to make that same shift for floor prep? Do the Pros of a product like Eco Etch or others outweigh to cons for the DIY consumer going over untreated concrete with a thin to medium build……
Hi Justin. You bring up some excellent questions, some of which we have been considering ourselves. Floor coatings have definitely evolved in the last 4 or 5 years, so I think it’s natural for concrete prep to evolve also. The DIY crowd had become more savvy as well about the importance of using a primer with high solids epoxy and as a result, proper etching instead of grinding has shown to work well. Furthermore, the newly popular high-end polyureas you refer to have excellent penetration properties that makes a properly etched concrete surface a great medium for adhesion. I believe vendors such as yourself who are on the front lines of this are the first to notice what is working best for the DIY applicator and what is not.
Does that mean that grinding is not necessary anymore? No – there will always be cases where it is the better alternative. However, with the advent of newer and safer etching products that have proven to be effective, I think the pros of etching with the right product have begun to outweigh the Cons of not grinding.
(For our readers: Justin Krauss represents one of our valued sponsors, Garage Flooring LLC.)
I would agree that grinding is still an important consideration, especially for those using 100% solids materials or even a high solids primer. We also suggest it for floors that need a little TLC, may have been sealed, basement applications and applications where getting the floor wet is disadvantageous.
If however, our customer is using a Polyurea on an unsealed floor in relatively good shape, etching is almost preferable.
If I have recently sealed my garage floor with a sealer (about a year ago), will this be a problem for applying epoxy? I was concerned about the pitting that was occurring due to the salt coming off the car during the wintertime. The sealer is not shiny, but it is a high quality product that Nuway recommended for this problem. Will grinding or etching properly prepare the surface even though I have applied this sealer? Also concerning the pits, is there a product that can be applied to the pits to smooth them out?
Hi Kim. Yes, you will need to grind in order to remove the sealer at the surface and provide a proper profile for the coating. You cannot acid etch. The reason is that the sealer prevents the acid from reacting with the minerals in the concrete since the concrete is sealed. We have an article here that details materials and methods for fixing pitting. What coating product do you want to apply?
I would like to use an epoxy coating of some sort. I was more concerned with properly preparing the surface at this point. If you have a recommendation to what epoxy product works well, I’m all ears.
If you are going to put the work in to properly grind your garage floor, Kim, then we would recommend staying away from your typical DIY home improvement store epoxy kits. These are all thin mil coatings that may require an additional coat to get good color uniformity since the concrete will be absorbing more of the coating. Plus, they typically wear out much sooner than higher quality coatings.
We would recommend a high solids epoxy kit that contains an epoxy primer, high solids epoxy base color coat, optional color flakes and a high performance clear coat. These are more expensive than the home improvement store kits, but they will last 15 years or more. The nice thing about clear coats is that as they age and wear, you can always rough them up to add another coat of clear to make your coating look brand new. Many of the sponsors on our website carry these type of coatings. You can visit this page here find links to them.
So I rented a diamond grinder and ground down the concrete floor of my garage, however I don’t think I did a good enough job. It looks like some spots are still smooth. Is it ok to acid etch after using the grinder?
That’s not uncommon, CJ. Do a water drop spot on those areas to test. If the concrete turns dark immediately and absorbs the water in less than minute then you are good to go. If the water just sits there and takes its time to penetrate, then you either need to regrind or etch.
Do you have any Acid or Enviro friendly Etch Products from your sponsors that you can recommend? I used two bottles of Klean Strip Green Muriatic Acid (90% less Fumes) but I’m starting to second guess if I have the “gritiness” I need from using just that. It feels close to a 150 grit sandpaper (I’ve heard recommendations of getting closer to 80 grit sandpaper feel) Also, If you use a commercial grit epoxy kit with primer are you able to get away with a less “gritty” etch?
Hi Stevie. It’s awfully difficult to determine how gritty the concrete is with your fingers. Have you done a water drop test? If you place a few drops of water on the concrete in various areas, it should immediately turn darker and absorb the water fairly quickly. If it doesn’t then you have more etching to do. If you feel you need to etch more and you are looking for safer alternatives, Acid Magic is a product that one of our sponsors that sells commercial quality coating systems uses with good success. You can find it here on Amazon.
I have a new concrete garage floor which had a deep penetrating sealer applied to it. Will grinding work to prep the floor for either epoxy or polyurea coating?
Hi Elizabeth. Yes, grinding would be required to remove the repellency of the sealer at the surface. When done, check the concrete with a few water drop tests in random areas. The concrete should immediately turn dark and absorb the water fairly quickly.
Concrete floor in horse barn has become slick over many years use, can I use muratic acid to rough it up?
Hi Lee. If the concrete is fairly clean without anything to block the acid from coming in contact with it, then yes, it will add some texture to the concrete. However, if there is a film, oils, or other contaminants that can block the acid from coming in contact with the minerals in the concrete, then it will not work very well.
Thank you, do I need to put anything on after muriatic acid, some say a neutralizer (not sure what that means)
Yes, you need to properly neutralize it afterwards or it will continue on reacting within the concrete. This article here explains how to acid etch concrete.
Hi Shea. I am not certain what is wrong with my garage floor. This is new construction and I understand the floor was poured in cold weather (January in Illinois) and covered with plastic. Not sure if that has anything to do with the appearance. It is covered with brown spots and when washed, takes on a crackly looking surface with white and brown spots. I want to have a professional epoxy floor put over it and want to make sure that nothing is wrong with the concrete before I do that. Sounds like grinding the floor is the best way to go but you also mention a moisture barrier be put on. Would that be put on after the grinding is completed? What can I use for a moisture barrier? Also, is there a way I can send you pictures, to get your opinion? Any help would be greatly appreciated! TY
Hi Antoinette. The appearance that you describe of your slab is fairly typical for one cured under plastic in cold weather. There should be nothing structurally wrong with the surface if that is your concern. All professional garage floor coating companies and installers should grind the concrete before application. If they don’t do that, then don’t hire them. Moisture vapor barriers are large sheets of industrial plastic that are laid on the ground before the slab is poured. They prevent ground water from being absorbed by the slab for areas that have water table issues with concrete or for slabs that are laid below the surrounding ground level. They cannot be installed after the slab is poured. Any pictures that you want to send can be sent to [email protected].
Hi Shea, I want to grind my new house garage floor in order to prepare for epoxy. Looking at grinding with a 5″ angle grinder using Diamond Cup wheel. What type and grit to buy ? The available ones are like 30/40 grit or 60/80 grit for Medium bond floors. Please guide.
Hi Rohit. You need a turbo cup grinding wheel with 25-30 grit diamond segments. A 5″ wheel is rather small for an entire floor. We highly recommend a 7″ wheel and appropriate sized angle grinder for that method.
I have purchased a high end metallic epoxy system for my basement floor. I am still deciding whether to etch or grind. It is 7 year old concrete with no sealer. I will have furniture etc that will remain in the basement that will have to be moved around. I’m wondering how much dust gets created by the grinder? Also is it ok to use a little water to help keep the dust down while grinding?
Hi Travis. A grinder with a good working vacuum system will still create some dust. You will want to cover the furniture if you are worried about it . Do not even attempt to grind inside a closed room without a proper vacuum system. It’s also very important to wear the proper N95 respirator. If you introduce water, it needs to be flooded with it and rinsed often. Adding only a little creates a concrete pastes that hardens in the pores and is almost impossible to remove without further grinding.
Good evening. prepped the floor, etched twice, scrubbed it with a lot of pressure, and power washed it. Was getting ready to apply a sealer, because of the moisture that seeps through a couple of times a year, and the comments were to apply on bare concrete only. I have also painted the floor twice in the last 20 years (cant remember if latex based). During the scrubbing, hand scraping, and power washing all loose paint and residue has been removed. Bought the 2 1/2 car RockSolid kit at a big box store and I followed the instructions and advice of the sales person and yes he would put the sealer on before the epoxy. No mention of grinding. Should have come here first! Michigan weather.
Hello, We have a garage floor newly poured in September 2019 The contractor put a sealer over the concrete..started having cracks and then places where the very thin top layer is breaking out. Fast forward to now…the contractor has come back and rented a sealer removing machine I think…it only made swirls and did not take all the sealer off the floor. He said it was a grinder..but a grinder would have taking off the top layer of cement. He then scrubbed the floor..and then used the etching (citrus acid) …then said he will be back tomorrow to put epoxy. I did the water test..some spots soak in..others just sit on top. I know the sealer is not all off. Does a stronger acid need to be used? Is it safe if we have a well only 10 ft from the garage? Or is grinding the only way..? Thank you
Hi Carol. It sounds like they are trying to correct for a poor finishing job of the concrete, correct? The surface condition you describe is typical of a slab that is finished with too much water on the surface in order to create a smooth finish. The results are generally dusting and a weak surface layer. Do not let them apply anything over the concrete yet. It’s fairly obvious that the sealer is not removed and it will only lead to the coating peeling up. They need to use a more aggressive grinder or spend more time with the one they used. Also, make sure they are using a quality epoxy product and not a 1-part epoxy paint or DIY epoxy kit from a home improvement store. You will be experiencing hot tire pickup in short order, especially if they don’t remove the rest of the sealer.
Etching solutions do not remove sealers. They need to reach the bare concrete in order to react with it in order to break down the surface enough to open the pores of the concrete. A sealer will prevent this from happening and the etching solution will do nothing. That is why there are areas where the water still sits on the surface. In addition, citric acid etches are ECO friendly and will not affect the well. The draw back to them is that they are also the weakest type of etching solution to use. Muriatic and phosphoric acid solutions are much stronger, but they still will not work if a sealer is present. Unlike citric acid etches, these type of acid solutions need to be neutralized at the surface first before being rinsed off.
Thank you for your quick response. I think they just want to get done and then out of here…I will NOT let him put anything else on the floor…If I have to finish it myself. I will go and rent a grinder and grind it …can I use water with the grinder right..so not so much dust. Then I let it dry a few days…then apply the epoxy. Right? This was supposed to be my husbands dream garage. We had a lift installed. He is disabled and can not do this job of emptying out his garage again. So we need to get this right. We do have the Lowes epoxy in the bag that you mix and probably can not take it back as the contractor used the etching bags…This is a mess. But..first thing first and that is to grind the concrete. Thank you for your help..
Since this is your husband’s dream garage, have you considered hiring a professional garage floor coating company, Carol? If it’s in the budget, they can easily take care of the sealer issues when prepping for a coating system. We have an article here about how to choose one. The reason I bring this up is because the single coat epoxy product from Lowes is a low budget product that is not going to provide the looks and long term wear that is typical of quality commercial products. It will begin to look worn in 4-5 years and may experience hot tire pickup. If you have the means to accomplish this yourself, there are much better products that we can suggest.
In terms of the grinding; Yes, you can use water to keep the dust down. However, it can create quite the slurry and it is imperative that it be kept wet and rinsed well as you go along. If not, all the concrete dust will settle in the pores and be a real bear to get out. It’s actually less messy and easier if you use a grinder with the proper vacuum equipment.
I have an epoxy floor that is in great shape (i.e., no delaminating) and would like to apply a new epoxy coating with flakes and a clear coat.
My question is should I use a wet or dry sanding method? Also, what grit sandpaper? Thank you.
Hi Greg. Use a dry sanding method. Clean and degrease the surface first. 120 grit sandpaper is recommended. We have an article here that talks more about recoating old epoxy.
Hi,
I am prepping to coat my garage floor. I have degreased, scrubbed, and buffered my garage concrete floor with a concrete prep tool I rented from Home Depot. I’ve noticed an increased amount of fibers sticking up on my concrete and was adviced to not use the acid etch that comes in the Rock Solid Rustoleum due to safety reasons with it interacting with the fibers. I wanted to etch one more time with the etch that comes with the kit because I am not confident I etched properly the entire floor since it was my first time using it, and it is hard to operate. I also noticed some areas where water would pool and took longer than a minute to absorb. There is little to no information on this problem and if it is truly a safety hazard. What is your recommendation ?
Hi Ema. You have a lot going on here. It sounds like you have fiber reinforced concrete. The etch that comes with RockSolid is a citric acid etch and is mild compared to a muriatic acid etch. There is no known safety issue with etching fiber reinforced concrete that we are aware of. However, etching is not recommended because it exposes the fibers at the surface. When that happens, all those little fibers stand up on end and poke out of the coating when it’s rolled on. This is why grinding is the preferred method. It will usually cut the fibers off at the surface if done correctly. If you grind and then etch it will expose the fibers. If you are still having issues with them, then the proper way to get rid of them is to burn them off with a propane flame torch. They look like this here. You just run the torch over the surface just long enough to burn the fibers away. It doesn’t take too long to do.
If you have water pooling on the surface that does not get absorbed well, then those areas need to be profiled more by either etching or grinding. You mention that you didn’t think you etched properly because it was the first time using it and it was hard to operate. Are you referring to grinding and not etching?
Hi Shea,
Very helpful posts. Thank you for doing all you are doing to help DIYers like me. I am convinced that grinding is better than acid etching but I still want to try an easy option first. I am thinking of using vendor supplied etch acid powder first and then Klean Strip Muriatic Green acid from Home Depot to etch the floor. Will etching twice (or may be 3 or 4 times) with acid give me a CSP2 level roughness that we can get from ginding? Should I be concerned about too much acid washing if certain portion of the acid piercing through the concrete pores and damaging the underlying concrete layer or rebars?
Hello Vijay. What coating product will you be applying?
Hi Shea,
I will be using Epoxy-Coat 100% solid Cycloaliphatic epoxy. I am also going to be priming the etched concrete floor before epoxy and then clear coat. Thanks for your help!
Epoxy-Coat uses a phosphoric acid powder, Vijay. It’s stronger than Klean Strip. You should not be doing multiple acid etches because it will cause the surface to deteriorate and crumble. In addition, the acid solution will work it’s way deeper into the concrete where it can’t be neutralized properly. It can remain active for months and do damage. Only if you have a very hard and smooth surface that doesn’t provide the proper profile after the first etch should you being etching twice.
Thank you Shea. I was thinking of acid etching today. The highest temp today is going to be 43F at around 2pm. Is it okay to acid etch at this temperature? Forecast for the highest temp in the next 3 days is between 50F and 58F, and I want to take advantage of higher temps to prime, epoxy and clear coat on three consecutive days. And, then there is 30-40% chance of rain for 2 days. If I don’t take this window of opportunity, I might have to wait until April to lay down my new floor. What do you suggest? Get it done now or wait till April? Thanks.
Hi Vijay. You can acid etch at most any temperature as long as it’s not below freezing. However, the weather is too cold to be applying epoxy. Epoxy requires a minimum of 55-60 degrees for a few days to cure properly. When the temperature drops below that, the chemically activated curing process stops and may not restart. Also, slab temps are important as well since the coating will quickly adopt the temperature of the surface. If it’s been cold, then the slab temps will be cooler than the air when it warms up temporarily. The spring would best if you don’t anticipate a longer warmup for your area.
Thank you for your sage advice. Really appreciate it. I’ll wait till April.