Can you jet wash sandstone?

Sandstone is a sedimentary rock formed from compacted grains of sand and mineral particles. Its natural riven surface texture, warm colour range and relatively accessible price point make it one of the most popular choices for outdoor paving across London. Like all natural stone, sandstone varies considerably in density and hardness between varieties. Some types, such as harder quartzitic sandstones, are relatively robust. Softer varieties, including certain Indian sandstones and paler buff-coloured types, are notably more porous and more susceptible to surface erosion from jet washing. The universal principles that apply across all sandstone types are: low pressure, wide nozzle, 40 to 50 cm minimum distance, pH-neutral cleaners only and no acid or bleach. Understanding the chemistry behind each of these rules prevents irreversible damage.

The properties that make sandstone vulnerable to damage

Concrete is a manufactured, dense composite in which the aggregate is held in a Portland cement matrix. Its surface is hard, relatively non-porous and resistant to high-pressure cleaning at appropriate distances. Sandstone by contrast is a natural stone composed of sand grains bonded together by natural mineral cements, primarily silica, calcium carbonate or iron oxides depending on the stone type. This grain structure makes the surface inherently more porous than concrete, which means cleaning products and water penetrate the stone body rather than remaining on the surface. It also means the bonding between individual grains at the surface is weaker, and high-pressure water can erode the outermost grain layer, leaving a roughened, more porous surface that collects dirt more readily and is more susceptible to further deterioration.

The characteristic colour of sandstone comes from the iron oxide and mineral content of the grains themselves. Bleach attacks these iron compounds and alters the colour chemistry permanently. Acidic cleaners dissolve the calcium carbonate mineral cement that bonds the grains together in many sandstone varieties, creating surface etching and weakening the grain bond. Neither of these effects can be reversed once the damage has occurred.

How to jet wash sandstone correctly

Products that permanently damage sandstone

Vinegar and citric acid cause surface etching of the grain cement in many sandstone types. Bleach permanently discolours the iron oxide minerals in the stone. Standard brick cleaners and patio cleaners containing hydrochloric or phosphoric acid attack the calcium carbonate grain cement and cause permanent surface degradation. Most commercially available "patio cleaner" products from DIY retailers are acid-based and are not appropriate for sandstone. Always read the product label carefully and confirm pH neutrality before use. If there is any doubt, test on a concealed area and wait 24 hours before assessing the result. The same applies to any rust remover, efflorescence remover or general-purpose masonry cleaner that has not been specifically confirmed as safe for natural sandstone.