Abstract: A granular expanded mineral material, e.g. vermiculite, is caused to move in the form of a free flow (28) through a mist (46) produced by an atomizer (29). A waterglass solution and a hardener therefor are supplied through conduits (32,43) and (33,42) respectively, to the intrior of the atomizer (29) for co-atomization to produce said mist (46). A small amount of hydrophobizing agent non-emulsifiable in the waterglass solution is introduced into the stream of waterglass solution in the conduit (32,43). An in-line mixer (53) is built into that conduit immediately ahead of its exit (45) to the atomizer. By passing through the atomized mist (46) the grains of the expanded mineral material are individually coated with a film of waterglass solution and hardener with hydrophobizing agent finely distributed therein. The film-coated granular material is immediately thereafter pneumatically conveyed to a place of deposition on a surface, e.g.