The Proper Way To Clean & Seal Exterior Slate or Flagstone

SDSC 11 Comments


After Cleaning and Applying Enhancing Sealer

Material: Flagstone: Exterior Patio: Approximately 2000 sq/ft

Customer Request: Bring out the color of stone without adding any shine.

San Diego Stone Care Exterior Stone Recommendations: The number one observation I see on almost every exterior stone application is what I consider, not necessarily the wrong sealer, but the wrong type of sealer that will cost you $1000.00’s of dollars to maintain over the years to come, applied to exterior stone applications. The reason I say it’s not the wrong sealer is because when these types of sealers are used on interior stone they usually work beautifully with no problems.

After application of Penetrating Enhancing Sealer

First, let me describe the type of sealer that is usually applied to exterior stone & why… and then I will describe the type of sealer that I recommend, which will not only make your patio areas look beautiful, but the sealer application will last longer & it will be less expensive to maintain over the long haul. The type of sealer that is usually applied is called a “Topical or Coating Sealer” which performs just like it sounds. When you apply this coating sealer some of the product does absorb into the stone and some of the sealer lays on top of the stone. When you apply more coats you are building layers on top of the stone…hence you are making a protective layer of sealer that sits on top of the stone…OK that sounds good so far right? In addition, especially as you add more coats the sealer will leave anywhere from a matte to a high gloss shine, which I know a lot of people like that look. Now these coating sealers also bring out more detailed color of the stone. The reason you see these coating sealers on almost every exterior stone installation, I believe, is because of this. Exterior stone installation is quite often done by a landscape contractor not a tile contractor & this is just fine I have seen some beautiful installations. But for whatever reason landscape contractors seem to like using these coating sealers, maybe because these are the same sealers they use to seal stamped and colored concrete for protection, and what is good for the goose is…well you know the rest. (By the way you will have same issue with your stamped concrete with these coating sealers but these coating sealers are the only thing that will protect your concrete…not true with natural stone)

Where is the Problem? A couple areas: One the fact that they are a coating and shiny can present a problem of being slippery especially when water, like in pool areas, gets on top of the stone. Secondly…UV sunlight & water both affect coating sealers in a negative way; actually sunlight makes all sealers less effective over time. But the real problem is water not only when you have standing water, like after a rain or from ones sprinklers, but also water that is below the stone…underground. You see coating sealers act like a lid on a can of paint, although many manufacturers state their sealers are breathable…this is not enough to prevent water vapor damage…so water that comes from below the coating is now trapped and begins to deteriorate the sealer. You will know this is happening, especially around the edges of your patio or areas where there is not much sunlight, because your stone and sealer begin to take on a white opaque look to it. So…we will get the call when someone sees this and they say: “My stone needs to be cleaned” Sorry…but cleaning alone is not going to fix your permanent problem. Last and the biggest & most expensive problem…what needs to be done to fix this damage?  Can I just add  more coatings to improve the look…yes you can but you will not improve the look. You will only make a damaged patio look shinier.  Can I just patch apply sealer to the most damaged areas…yes you can & you will have the same results a shiny damaged looking stone.

Before cleaning and before enhancing sealer

The San Diego Stone Care Solution: The only solution which will bring back your stones original look is to strip off as much of the old coating sealer as possible, which is a time consuming, costly process. Why? These coating sealers require using very smelly dangerous & costly stripping solutions, by hand, one tile at a time, to remove as much of the coating sealer as possible. (You will never remove all the sealer as some will stay trapped below the surface of the tile.)

Enhancing sealer applied. Lower right side of picture has no sealer. You can really see the contrast here – beautiful!

So once you have all the coating sealer off the stone and it has been allowed to dry properly…now we can apply a much better longer lasting “Penetrating Enhancing Sealer”. (If you have a new installation that has not been sealed yet this is an even a better situation to have.) How is this “enhancing sealer” different? First…it’s a penetrating sealer, which means, it is absorbed, into the stone and grout and protects the stone from below. (There is no sealer left on the surface of the tile….If there is any sealer residue on top of the stone, your stone care company incorrectly installed the enhancing sealer…we see this almost every week…please let San Diego Stone Care do your stone maintenance!) Second…the sealer also “enhances the natural colors” of your stone WITHOUT adding any shine…this is a good thing – no slippery stone) . What makes the enhancing sealer a better sealer application? Although every sealer is affected by the suns UV  rays, penetrating sealers seem to last longer than coatings since they are below the surface verses coating sealers which are more exposed to UV since they sit on top of the stone. Also…penetrating sealers are a better breathable sealer…which means these do not trap moisture that comes up from the ground but allows the water vapors to transmit safely through and out the stone and grout. (No white hazing looking tiles) Lastly…enhancing sealers do not need to be removed at all in order to re-apply more sealer down the line and they do not need to be applied as often as coating sealers to be effective. So re-applying enhancing sealer in the future is as simple as giving you stone a good cleaning, allowing it to completely dry, and re-applying the enhancing sealer. This is actually easy enough that with some direction from me some of my customers re-seal their own patios. (I would doubt any customer would want to attack removing a coating sealer…we don’t even like doing it.)

FYI…there are also “Clear Penetrating Sealers” which perform exactly as do the enhancing sealers except “Clear Sealers” do not add any color to your stone or grout…If you like how your stone looks without any sealer then a Clean Penetrating Sealer is the right sealer for you.

After Applying Penetrating Enhancing Sealer


Comments 11

  1. Bev

    I am very interested in your work/product but I live in Canada. How can I purchase a clear penetrating enhancing sealer in London, Ontario, Canada? I have a hot tub in a solarium with a flagstone floor. The floor needs to be cleaned and sealed (never has been cleaned or sealed yet)

  2. Post

    We are not currently set up to ship products to Canada, however, you can order directly from Manufacture Bright Stone International. Just make sure to mention that Tim with San Diego Stone Care referred you directly to them.

    Thank you, Tim

  3. Paul

    I went to the brightstn website but which product do I get for flagstone. Which one is the enhancing sealer?

    thanks, Paul

  4. Post

    Hi Paul, Call Brightstone and they will advise you on the best enhancing sealer. They do not put all their products on the website. If you need any more assistance please feel free to give me a call direct.
    Thank you, Tim

  5. Ed

    I have a flagstone patio with a workshop/garage below. I get a little water penetrating through the structure in heavy rains. The flagstone sits on 6″ of concrete which sits on galvanized decking and steel girders. I have used a stone enhancer as you recommend, but doubt that can seal any cracks between the stone and mortar. Would a topical sealer create a waterproof layer, and if so, what would you recommend? Thanks for the help.

  6. Post

    Hey Ed

    A topical sealer will not create a waterproof layer, actually, no stone sealer, including an enhancing sealer, will not create a waterproof layer they just are not made to do this. In addition, not only will the topical sealer not create a waterproof layer it will actually be a higher cost maintenance issue especially with rain & snow. What should’ve happened during the original installation of this system would have been to apply a “true waterproof barrier” either on top of concrete… assuming also that concrete has water run off sloping or drains. This being said I do understand that this does not help your current situation…so I will give you a recommendation that really won’t “solve” your problem …nor do I like doing this…but is MAY slow down the water.

    First, I hope you flagstone has some kind of water run off system in place so that water is not just sitting on top of your flagstone. As I first mentioned I do not like recommending coating systems outside for a couple of reasons. #1 high maintenance and these products need yearly re-coating or they deteriorate quickly. They are also extremely slippery in wet weather which can increase slip & fall. However, if you MIGHT reduce the amount of water that penetrates the stone by applying 3-5 coats of a coating sealer. (Glaze N Seal makes a very durable “lacquer one” which is a bit difficult to work with but does hold up well outside. (Call manufacturer for proper application). As I stated coating sealers are not waterproof barriers but logically by having several layers of basically a clear coating it will be more difficult for the water to penetrate as easy. This is not necessarily a good solution but more of what I will call a “band aid solution” that may help and is more economical that replacing the current installation with a proper waterproof membrane…which is what would be the best solution.

    Hope this was helpful


  7. Tonie

    Tim. I have a flagstone walkway and it appears to have a glaze on the stone from improper cleaning the lines of cement. What can i do to get the natural color on my stone to pop out from the whitish looking film.
    Warm Regards,

  8. Sam St. John

    You are commenting on sealing a flagstone patio but what about the sealer the installers use to seal the stone before grouting. What kind of sealer is this? They say they have to seal the stone so the epoxy grout will not stain the stone. This looks like it will not allow the grout to grip the stone because the sealer is a barrier between the stone and the grout. I am a little confused?


  9. Post

    Hi Sam, Sorry for the delay. Good questions! I too am a little confused? Interior or Exterior Application? Why are they grouting your flagstone with an epoxy grout? Don’t get me wrong nothing wrong with an epoxy grout, actually, it the color holds up extremely well with epoxy grout, as well, it has excellent sealer properties…its just a bit unusual for a flagstone installation. But if they are using an epoxy grout then they are correct to pre-seal to allow for easier grout clean up during the installation. It will also help reduce or eliminate any “color enhancing” that the epoxy grout will cause if you do not pre-seal. The resin in the epoxy will actually permanently darken the any part of the flagstone it comes in contact with, especially the edges of the stone…which will then cause “picture framing” which basically means the edges are darker in color than the rest or middle of the stone. These sealer recommendations are for exterior application if your installation is interior there is one more sealer option, “clear coating sealers”…which I personally do not normally recommend for outside applicatons. (See other blog answers for reasons why)

    The choice of sealer you choose to pre-seal with should be the same sealer your are going to apply after the grout installation. “High Grade Clear Penetrating Sealer” if you DO NOT WANT TO DARKEN. “Enhancing Penetrating Sealer” is you want to DARKEN THE FLAGSTONE.

    As far as the sealer falling into the sides of the stone, yes the sealer & grout manufacturers all recommended to prevent “excess”sealer from running into grout joints and I am a firm believer in following manufacturer’s recommendations. However, glass & porcelain tiles are almost completely non-porous, and we grout these material all day long with no problems. And even if some of the sealer did fall into your flagstone joints this would not even come close the same lack of porosity that a porcelain or glass tile has. Basically, avoid getting sealer in joints but its just not that big of a deal.

    Sam, I hope this has been helpful if you need anymore details please let me know via the blog of simply give me a call on my cell phone: 760-585-5406 Tim Connelly

  10. Post

    Hi Tonie. I am not sure if I am quite understanding your situation? When you say “glaze” might this be a “coating type sealer?” Does it have a sheen or shine to it? The reason I ask is because coating sealers are the number one reason that stones, flagstones, appear white over time…water gets under the coating and causes a milky opaque look, as well UV from the sun. I do not know what you mean by “improper cleaning the lines of cement…do you mean the grout joints? The only other thing that can cause a whitish look is “efflorescences” which is a white powder mineral deposit that is caused by excessive water from rain, sprinklers, etc. the water evaporates and leaves behind a white powder. Could you perhaps send me a few pictures.

    #1 – lets assume you have a coating sealer that has turned white. See my article on “Proper way to clean and seal slate and flagstone” There are two ways to fix this the first and best way is to finc a chemical stripper to remove all the old coating off the top of the flagstone, Home Depot / Lowes has epoxy coating strippers in gallons by Jasco or Kleenstrp. This are hazardous materials so please read all instructions for safety. (Quite honestly stripping off coatings really should be done by a professional for not only is it hazardous but it is a real nasty job.) But lets say you get all the old coating sealer off…after this just follow the instructions in the article I have already mentioned.

    #2 – Now lets say you do not want to remove the coating. Well you could clean the flagstone really well and rinse with clean water…allowing it to dry for at least one day. Then you can apply an “enhancing penetrating sealer” (The two that I like and use are Aqua-Mix Stone Enhancer & Miracle’s 511 Enhance & Seal) BUT BEFORE YOU SEAL THE WHOLE AREA FIND A TEST SPOT WHERE THE WHITE IS SHOWING THE MOST AND APPLY SEALER ONLY HERE. LET IT DRY FOR A DAY AND SEE IF YOU LIKE THE FINAL LOOK. This will not get rid of the whitish haze but it may just hide it enough to your satisfaction. This method is not my preferred method but it usually will improve the look of your stone without the expense and danger or stripping off the old sealer first.

    Hope this has been helpful and please feel free to contact me if you have any more questions or need further clarification.


  11. Tonie

    yes, My flagstone was put down with cement in the joints. when smoothing the cement out the cement residue was smeared up on the stones. the areas was cleaned with soft sponges and water but there was still white residue left behind. Now, the rain and sun may have added the to problem but there is a white film left on the stone.
    I have cleaned each stone with Muriatic Acid and the whiteness went away some. I can truly see the color of each stone.

    Now, i want to maybe clean one more time and seal it to keep the proper looks of each stone.

    What do you suggest?

    Warm Regards,

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