How do I remove cactus sap stains from flagstone?

Q. How do I remove cactus liquid from flagstone? The cactus was trimmed and the liquid splattered on the flagstone. It was not rinsed off right away and left several stains.

A. You’re not alone. Cactus sap can surprise people with how quickly it soaks in to flagstone and how stubborn it becomes to remove once it dries. Flagstone is porous, so any organic material that hits it and isn’t rinsed immediately can work its way below the surface. Cactus liquids contain sugars and natural pigments that darken as they oxidize, which is why the stains show up even after you’ve cleaned the surface. 

The good news is that organic stains on stone are usually treatable. The chemical many stone pros reach for is hydrogen peroxide in a higher, salon-grade concentration, typically 30 or 40 volume. This is the clear kind used for hair developers, available at beauty supply stores. The common brown-bottle peroxide from the drugstore is only about 3 percent and generally too weak to break down deep organic staining in stone.

Test in an inconspicuous area first. A straightforward method is to apply the 30–40 volume peroxide directly to the stained areas and let it sit long enough to penetrate. If your flagstone is a vertical application, you can soak a few layers of paper towel or a white cotton cloth with peroxide and lay it over the stain to keep it from evaporating too quickly. This creates a basic poultice effect that helps draw the stain out. You may see bubbling as it reacts; that’s normal.

Once the dwell time is up, rinse thoroughly with clean water and reassess. It often takes a couple of rounds.

If the stains are older or deeper, use peroxide to create a poultice to draw the stain out of the stone over time. For detailed, step-by-step instructions on how to mix and apply a poultice correctly, check out our free Stain Management App.

Here are a few cautions so you don’t accidentally create a new problem. Test the peroxide in a small, inconspicuous area first. Some flagstones, especially softer or very light-colored ones, can lighten slightly if left in contact with peroxide too long. Also, be careful around adjacent grout, mortar, or nearby plants. Peroxide at this strength can irritate skin, so gloves and eye protection are a good idea.

If you try a couple of peroxide applications and the stain still won’t budge, that could mean it has penetrated deeper than a DIY treatment can reasonably reach. Professionals have access to commercial-grade poultices, special equipment, and stone-safe cleaners that can speed up the process. To find a vetted PRO in your area, visit www.surfacecarepros.com and click on Find a PRO. In addition, a PRO can refinish the stone, if needed, and apply a sealer.

ABOUT THE AUTHOR Fred Hueston is the Chief Technical Director at SurfaceCarePROS.com and Director at StoneForensics.com. He is also the author of Stone and Tile Restoration: The Manual, a comprehensive online manual for stone and tile restoration contractors. [Learn more about Fred.]