Abstract: An improvement is proposed in the assemblage of a cover-packing bult-up body for a magnetic hard-disc unit to be freed from the troubles caused by dusts or contamination of the gap space between the magnetic recording disc and the magnetic head as well as by the poor vibration-damping characteristics of the rubber packing between the covering member and the body of the casing. The improvement comprises, instead of using a pre-shaped rubber packing, the built-up body is assembled by using a curable liquid silicone rubber-based composition filled with fine hollow microspheres to form a packing on the surface of the covering member by the in situ curing of the composition applied thereto so that the rubbery packing layer has a spongy structure exhibiting good vibration-damping characteristics.