Ian - in the many discussions I've read here and elsewhere as well as conversations I've had in person with other platinum printers, this phenomenon seems to raise its ugly head more with certain papers than with others. I can't recall ever seeing this on any prints I've made on Bergger COT320 - have you ever tried it? I remember there was a big batch of Arches Platine a few years ago that had a really bad case of plague spots, but that batch seems to have worked its way through the system and this is no longer the problem, or at least much less than it used to be. I also recall folks finding pre-acidification of their paper to help with this - a bath in 5% - 10% Oxalic Acid for 10-15 minutes seems to be a general consensus, with a preference for the weaker dilutions of the Oxalic Acid. Let me be clear- this process is not needed for all papers, and it may or may not resolve the plague spot problem. I don't know about Buxton, but Fabriano Artistico EW is one that definitely benefits from pre-acidification. I haven't tried it yet but I'll put good money on Rives BFK also needing pre-acidification, as I've gotten wierd blotchy areas on gelatin-sized but not acidified Rives BFK I was planning to test with gumover printing.