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Stonework cleaning is a complex and generally fairly specialised job.

We tend to be involved with removal of paint from stonework or just cleaning it to freshen it up. Other than lime based paints which generally blast off quite easily because they are powdery, in most cases blasting the paint off soft stonework can abrade the stone more than you really want to do. If the stone is hard it may be able to withstand it. Soft stonework is much more vulnerable and if the paint is thick and rubbery the blasting process has to be used more aggressively to break through, and whilst peppering the coating the impact of the abrasive can be transmitted through the rubbery layer and start to degrade the soft stone behind. Eventually the coating fails under the blasting and the stonework exposed in patches. Removing these patches can be a real problem if damage is to be avoided, so alternative processes are preferable.

Chemical stripping first can sometimes help. This involves the use of paint removing chemicals, to break down the bulk of the paint so that the subsequent blast cleaning does as little work as possible. It is a very messy job and involves water washing too, but careful sheeting usually contains it. Once you have got rid of the rubbery substances, gentle blasting will usually finish it off with minimal abrasion. Water injection is sometimes used to further soften the cutting action.

Strong chemical cleaning of stonework is not something we get involved with generally, it involves the use of aggressive and sometimes very dangerous substances. There is a concern about chemical cleaning of stonework on buildings, referred to as 'Clean and Green'. This relates to the fact that the chemical treatments and acids used on the stonework remove the salts in the stonework, allowing the clean stone to absorb much more water than the original stone. This allows verdigree to get a foothold and your nice clean building turns green in a very short time.

There are other blasting and cleaning systems out there but they are all much alike, just little refinements. Soda blasting, literally blasting with Bicarbonate of soda is a gentle process that would seem, on the face of it, to be a harmless process. However, that is not the case and the residues left on / in the stonework can be very damaging.