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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.