PURPOSE: To form uniform thick gaskets good in stickiness to caps for bottles or jars by lining the caps with a liquid gasket compsn. composed. of an aq. rubber dispersion containing a gelling agent to heat the lined caps for a short time.

CONSTITUTION: Caps of bottles or jars are lined with an aq. rubber dispersion prepared by compounding a rubber latex, pref., latex containing at least 50 wt.% of natural rubber and 50 wt.% or less carboxylated styrene/butadiene rubber (A), a filler (B) and a gelling agent (pref., an ammonium complex of polyvalent metal ions) (C) and other additives (an emulsifier, a dispersion stabilizer) and the lined caps are heated at 140-220°C pref., at 150-180°C for 1,4-10 min pref., for 1-3 min to form a gasket with a thickness of 0.5-4 mm, pref., 1-4 mm within each of the caps. The gasket compsn. composed of the dispersion is liquid but the gelling agent gels the compsn. during heating before the evaporation of water is generated to improve the mounting to the caps.