First my sympathy to Mike . Well here are my thoughts on the armies to dust. Twice in the 70s there was an oil shortages this led not only to the cost of plastic rising but also great shortages. Plastic was recycled but as the company I was working for found out the plastic had to be washed to remove the release agent also some "cheap plastic pellets" we bought were low grade and full of impurities, The "oil" used had not been refined to a high standard. We also sold some very old waste plastic that we had classed as not to be recycled This I think could be part of the reason we now have these problems.
Thats very interesting JAH. That would indeed probably have had an impact as you say.
My experience was with a companys figures made in a rather soft, rubbery sort of plastic. At the time I used enamel paint, & they started to crumble barely a year after I painted them!! These figures were "modern day" so not involved in the 70s issues you mention. Whether or not something similar involving the oil content was at fault, I have no idea.
However, since painting some more of these same sets with the rubbery plastic, this time using acrylics, I have had no problems......not yet anyway!!!!