So you have cleaned your patio with you pressure washer. Although the majority of the dirt is gone there are still black spots all over the surface. What are they? Pollution, oil, algae? … There are lichen spots. Pressure washing alone will not remove the marks. It is especially common on Indian sandstone. So How do you remove black spots on a patio?
Indian Sandstone cleaning in Cardiff
How do you remove black spots on a patio?
The following patio was cleaned using chemicals and a low pressure hot wash. High pressure patio cleaning is not needed. A common thought is that when there are black lichen spots on your patio just turn up the pressure. Or use a larger pressure washer to blast the black spots away.
This is not a good idea!
By applying a chemical to the patio the algae is killed and lichen spots are killed and far easier to remove. This means the Indian sandstone cleaning can be carried out with low pressure. High pressure will damage the face of concrete slabs and destroy the pointing between the patio slabs.