Cleaning Stone Masonry
How Does Stone Get Dirty
All stone varieties used for building both residential and commercial structures are porous in nature. These include granite, marble, and limestone. The types of “dirt” can include salt, rust, asphalt/tar, hard water, silicone/sealant, carbon/pollution, oils, organic (mildew, mold, algae), and actual dirt. Depending on how the stone is oriented in the structure and its surrounding environment will determine the type of dirt it accumulates. For example, if one side of the building faces a freeway, carbon buildup from vehicles will be an issue. If the stone is around a parapet as a capstone, organics, dirt, and maybe asphalt/tar or silicone/sealant is probably on top of it.
Cleaning Stone: Mechanical vs Chemical
Cleaning falls into two categories, mechanical and chemical. Mechanical is just water and elbow grease, a stiff bristle brush and water of either low or high pressure. Chemical involves a specialized cleaner geared towards the type of staining on the stone along with water and agitation.
Chemical cleaners are made by companies specializing in cleaning, protecting, and maintaining masonry and concrete. Prosoco is one example. When I used to work in masonry restoration, our cleaning crew would use the Sure Klean line of their cleaners. These cleaners are specially formulated for many or the stains mentioned above. The only down side is that these are serious professional cleaners and can do damage if not used correctly. If you own a commercial building with a masonry facade that needs cleaning, hire a professional.
Cleaning Stone with a Pressure Washer
For most residential situations, mechanical cleaning works just fine. For this you’ll need a pressure washer with a fan tip and maybe a stiff bristle brush.
- Clear the area of anything you don’t want to get sprayed with over spray.
- Start in an inconspicuous area until you get the hang of it.
- Moisten a small area and attempt scrubbing with a brush. If it works, continue with this method.
- Try the pressure washer and slowly move the wand closer to the stone while removing the dirt. Stop and check to make sure the water stream isn’t damaging the stone. You only want to get close enough to just barely remove the staining.
- Slightly overlap consecutive passes. If working on a vertical surface start high and work down towards the ground. Let gravity take the dirt onto the area not yet cleaned. If on a horizontal surface just keep moving the dirt in the same direction so that it doesn’t wash back onto a previously cleaned area.
- After you’re all finished let the stone masonry dry for a few days.
- Seal the cleaned stone masonry with a Silane/Siloxane sealer. The sealer can prevent dirt and stain buildup for a period of years depending on if it’s horizontal or vertical surface. Vertical surfaces will be protected longer.