A container for a plastic substance includes an axially elongated sleeve defining a hollow space for holding the substance. An extrusion piston is slidably displaceable through the hollow space for extruding the substance out of a discharge outlet. The piston forms a closure for the hollow space. Before the substance is extruded from the sleeve, a sealing ring, inserted between the inside surface of the sleeve and the piston, prevents the penetration of air into the hollow space, and the loss of gases formed by the evaporation of solvents out of the hollow space.