Soft washing stonework

is a low-pressure cleaning method that uses a specialized, biodegradable chemical solution (typically a biocide and a surfactant) to remove organic growth (algae, moss, lichen) and dirt without damaging the stone surface. 

Key Principles

• Low Pressure: The solution is applied using a softwash system or a garden hose pressure (under 500 PSI), which is much gentler than a high-pressure washer. High pressure can etch, strip, or crack delicate natural stone, brickwork, or mortar joints.
• Chemical Action: The cleaning agents do the primary work, penetrating the porous stone to kill organic growth at the root and break down pollutants at a molecular level.
• Dwell Time: The solution is left on the surface for a specific dwell time (e.g., 15-60 minutes, depending on the stain severity) to ensure effectiveness before a gentle rinse.
• Long-Lasting Results: By killing the spores and bacteria, soft washing provides a deeper, longer-lasting clean than pressure washing, which only removes surface-level growth. 

Equipment and Chemicals

• Equipment: A softwash system typically includes a pump, hoses, and an application wand. This equipment is designed for low-pressure application. For heavily ingrained stains, a low-pressure steam cleaning system (like a Doff or Torc system) might be used in conjunction with or as an alternative to chemicals.
• Chemicals:
  • Sodium Hypochlorite: The primary agent used to kill mildew, algae, and moss spores.
  • Surfactants: Added to help the solution adhere to the surface, penetrate deeper, and lift dirt and other contaminants.
  • Biocides: Applied as a post-treatment to provide residual protection and inhibit future organic growth for an extended period (up to 3-5 years).
  • Avoid: Do not use cleaners containing acids (like vinegar or muriatic acid) on acid-sensitive stones such as marble, limestone, or travertine, as this can cause permanent damage and etching. 

Best Practices & Safety

• Know Your Stone: Different types of stone (e.g., hard granite vs. softer sandstone) may require specific cleaning agents or techniques. It is crucial to understand the stone's composition to avoid using inappropriate chemicals.
• Test First: Always perform a test clean on a small, inconspicuous area to check the stone's reaction to the chosen solution and pressure level.
• Protect Surroundings: Thoroughly wet nearby vegetation and cover delicate plants, furniture, and electrical outlets with tarps to protect them from chemical exposure.
• Wear PPE: Always wear appropriate personal protective equipment (PPE), including safety goggles and gloves.
• Control Runoff: Plan where excess solution will go to prevent contamination of soil or water sources. Neutralize excess solution before disposal.
• Avoid Harsh Tools: Use a soft nylon or natural bristle brush for agitation instead of abrasive metal brushes or scouring pads, which can scratch the stone.
• Optimal Conditions: Avoid applying solutions in direct sunlight or windy conditions, as this can cause the chemicals to evaporate too quickly or overspray onto unintended areas. 

For best and safest results, especially on large or historic structures, it is often recommended to use a professional with specific expertise in stonework soft washing.