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Staining of porous cladding materials such as stone by migration of fluids in sealants became particularly prominent in the 1980s, probably because of the number of projects built then. It has remained a problem for the unwary. The first problems were with oil and butyl-based sealants, followed by silicones. Some examples of staining have also been found with acrylics, polysulphides and polyurethanes.

Slightly porous stones such as slates and marbles appear particularly vulnerable. Once fluid is absorbed into the stone, migration can continue after the sealant has been removed.

Primers/sealers can prevent the effect. Newer formulations of sealant have been developed by several suppliers. Some of these are marketed as non-staining, used without primers.

While sealants obviously play a part, in some cases the introduction of a new type of stone creates new problems.

Non-porous materials can also be affected, with sealant migration across surfaces which then pick up dirt.

A few companies, such as Adshead Ratcliffe in Derby, have developed stain treatments, available as a service rather than a product.

More details of sealant use are given in a guide by Shaun Hurley of Taywood, with Stephen Ledbetter and Tony Sheehan of Arup - Sealant Joints in the External Envelope of Buildings, ciria Report 178, published this year, available from crc, tel: 0171 505 6622.

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