Hey
@Andystiletiling what about a poultice, u may have necessary chemicals already which means u can try straight away rather than leaving in to soak in more.
Not saying it'll work but might diminish it a great deal.
How bout this, couldn't remember the right chemicals so found this.....
Alkaline poultice cleaners and strippers are commonly used for cleaning or degreasing masonry surfaces and for paint removal. Sodium hydroxide is the most common alkaline cleaning agent in proprietary cleaners for a range of masonry substrates, including limestone, sandstone, brick and terracotta and is the most common ingredient in proprietary paint removers. Care must be taken in the use of sodium hydroxide based cleaners to minimise risks to the building and the user. Sodium hydroxide based cleaners and strippers must be neutralised with acid afterwash. Adjacent, dissimilar building surfaces must be protected and personal protective equipment worn by the cleaning operative. In the field of stone conservation ammonium carbonate is added to clay and clay/paper poultices to remove soiling from limestone. Ammonium carbonate is a less alkaline cleaner than sodium hydroxide. It works by reacting with calcium sulphate on the soiled surface to form calcium carbonate and soluble ammonium sulphate that can be rinsed off with water.
These 'active' or 'chemical' poultices are all applied to a pre-wetted surface to minimise penetration of the chemical into the masonry surface and covered with plastic film to prevent the poultice drying out. The cleaning additives in these mixtures chemically dissolve the soiling or staining which is held to the surface of the poultice, and then both the cleaning agent and the contaminant are removed with the clay. Rinsing with water and, where necessary neutralisation, follows to remove any soiling that remains on the surface and also to remove residues of the chemical cleavers.
Hope it helps mate.
