I was interested to read Colin Honess of Corus' letter in response to an earlier Hansom piece about staining on the National Centre for Pop Music in Sheffield ("Help, the mastic leaches!", 17 January, page 35).
The leaches in question must be long-lived and well-travelled.

In 1986, I investigated the cause of black staining to the stainless steel cladding on Britannic House, which, at something more than 30 storeys, was I believe at the time of its construction (circa 1968) the tallest block in London.

My vertiginous investigation, in part accompanied by the SAS as they abseiled down and the facade to string banners launching a BP publicity campaign (I should say that they were demonstrably bolder in their task than I) showed that the cause of staining to the otherwise unblemished stainless steel was the leaching of an oil constituent from the mastic joint sealant that spread across the surface of the cladding, attracted and retained airborne dirt, and as a consequence gave rise to the discoloration.

In the event, it was relatively easy to clean it off, subject to access, but of course any loss of constituent parts can only reduce the effectiveness of such a joint.

This is not to condemn necessarily the design, materials, workmanship or maintenance of such a system but simply to demonstrate that attention to detail should be employed in all these respects if the leachate problem is to be avoided.