Anyway, on opening the boxes and picking out the polystyrene chippings used to pack them, I discovered that some of the figures had become considerably 'hairier' owing to a nasty instance of lead rot. Some looked like iron filings attracted to a magnet while others had crumbled to dust. Happily, the vast majority are in perfect condition.
AFAICR all these figures would have been undercoated with a polyurethane varnish stained with Humbrol enamel matt black, painted in Humbrol matt enamels and then given a sealing coat of varnish.
That was my standard approach for 25mm figures in those days and I've never seen this problem with any other armies.