1 microns the microspheres usually have densities from about 0 03 g cc to about 0 7 g cc in the case of hollow microspheres of vinylidene chloride acrylonitrile the density is usually about 0 03 g cc the particle diameter is on the average about 30 microns and the wall thickness is about 400 to about 500 angstroms since in keeping with the process pursuant to the invention a relatively small quantity of hollow microspheres has to be mixed with the unsaturated polyester together with any other desirable additive such as hardeners stabilizers and polymerization accelerators as well as the active substance the manufacturing process is greatly simplified and thus has economic advantages the hollow microspheres may be added in very finely graded quantities and uniformly distributed in the resin it is also possible to prepare intermediate products of relatively large volume which may subsequently be reduced to their finally desired form through a mechanical treatment such as sawing among the hollow microspheres of inorganic material those made of glass are preferred because hollow microspheres made of glass are very much simpler to manufacture and therefore also more easily accessible than those of sodium borosilicate among the hollow microspheres of organic material those of vinylidene chloride acrylonitrile copolymers are especially preferred such hollow microspheres excel above all through the fact.