How to Clean Flagstone


Mark asked: What is the best way to clean flagstone? I have a light colored flagstone patio and walkways which get very dirty from the elements and mold (shaded areas). I have a power washer and have used strictly water, but it needs something better.

Flagstone provides a beautiful, natural-looking landscape and requires special care to protect it from damage. Here are some steps to get your flagstone looking new again and removing serious dirt and stains.

You Will Need:

  • Muriatic acid
  • Bleach
  • Water
  • Bucket
  • Plastic bristled brush or broom
  • Flagstone sealer (optional)

Steps to Clean the Flagstone:


  1. Mix 1 quarts of muriatic acid with 2 gallons of water in a bucket.
  2. Use the brush to apply the mixture to the stone and scrub well.
  3. Rinse COMPLETELY with water. Allowing the acid to sit on the surface will result in damage.
  4. Next, mix some bleach in with a bucket full of water. The amount of bleach needed will depend on the type of flagstone and the level of dirt build-up.
  5. Apply the mixture with the brush and scrub again until the surface is clean.
  6. Rinse again with clean water, ensuring that all of the bleach is removed.
  7. Allow the area to dry completely, at least 24 hours or more.
  8. Apply a good quality sealer to the surface following the manufacturer’s guidelines. This will protect the surface and make future cleanings much easier.

Additional Tips and Ideas

  • For general cleanings, a power washer with plain water is best.
  • Avoid using any acidic or scouring cleansers as they can damage the surface of the stone.
  • To get rid of mold and mildew, apply boiling water to the surface and scrub with a plastic brush.



  1. Paul says:

    I think it might be wise to mention the fact that muriatic acid is an extremely dangerous chemical, and that anyone using it might want to have the necessary equipment needed (rubber boots, heavy duty rubber gloves, vinyl-coated cover-alls, full-face protection with a respirator, etc.). Your article gives absolutely no indication of this.

  2. Max says:

    In addition to the safety precautions, mixing chlorine bleach with an acid will result in the release of chlorine gases that are toxic, resulting in burning of the lungs in high enough concentrations. Be very careful to wash the acid away thoroughly before using the bleach solution.

    Good luck!

  3. Stephanie says:

    I would hesitate to use a product as dangerous as muriatic acid. I use a green product called Concrobium to clean mold off my large flagstone patio. Apply, let dry, and then pressure wash or scrub. Does a fantastic job.

  4. Keith says:

    You also want to be careful not to let the muriatic acid wash into your plants because it will kill them dead. Also, acid splatter can damage some metals, etc. Be careful!

  5. Joseph says:

    All the comments say not to use muriatic acid or be careful, but they do not say how to remove a mortar stain off flagstone. I used a filler in my joints on the patio called Sakrete Paver Set Polymeric Sand; it left a gray dust edge on all the flagstone. Can anyone help?

  6. Maura says:

    I have the same problem as Joseph where the polymeric dust left my beautiful flagstone looking all gray. Any help for this?

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