Stone Cleaning Cardiff

Identify the stone before touching it

Everything about the method follows from what the stone is, so that is where we start:

• Sandstone, including most Indian sandstone. Porous, relatively soft, holds black spot lichen badly. Tolerates mild acid but scours easily under pressure.
• Limestone, including Portland and many pale patio stones. Calcium carbonate, which means acid dissolves it. Any acidic cleaner etches the surface permanently.
• York stone. Hard, dense sandstone. Takes more than most, but old flags are often thin and bedded on lime, and the joints are the weak point.
• Slate and granite. Hard and largely inert, so the risk is mechanical rather than chemical.

Why acid on limestone cannot be undone

Acid and calcium carbonate react. On limestone that reaction is not cleaning, it is dissolving: the surface goes dull, slightly rough, and often lighter in patches where the product sat. There is no product that puts it back. This is why a general purpose "patio cleaner" that did a lovely job on a neighbour sandstone can wreck a limestone terrace in one application.

Black spot is a different job from dirt

The stubborn black dots on sandstone are lichen, biologically rooted into the surface rather than sitting on it. Pressure takes off the top of each spot and leaves the root, so it returns within months, and the pressure needed to actually cut them out takes stone with it. Black spot needs a specific treatment left to work over time. Some of it clears in days, the rest fades over following weeks, and we say that plainly rather than promising a same-day result we cannot deliver.

Our method on natural stone

Soft wash by default. The stone is pre-wetted so nothing can be drawn deep, treated with the chemistry appropriate to that specific stone, given time to work, then rinsed at low pressure with a wide fan or a rotary head set to float rather than bite. Where a stone is sound and heavily soiled we may use hot water, which does more work at a given pressure and therefore lets us use less of it.

Joints get checked first. Old flags on lime mortar lose their pointing before the stone shows any distress, and washing out the joints is how a clean turns into a repair.

Sealing, and when not to

An impregnating sealer slows staining and makes the next clean easier. It has to be breathable, and the stone has to be genuinely dry before it goes on, which in Cardiff usually means a return visit rather than the same afternoon. We will not seal damp stone to save a trip, because trapping moisture under a sealer causes a cloudy bloom that is harder to fix than the original dirt.