An elastomer sleeve is applied to a wellbore tubular. The elastomer sleeve, which defines a circumference around longitudinal bore in a longitudinal direction, is brought in an elastically stretched condition by applying a stretching force to the elastomer sleeve. While the elastomer sleeve is kept in the elastically stretched condition, it is moved in the longitudinal direction over the wellbore tubular to a selected position on the wellbore tubular whereby the wellbore tubular extends through the bore. The elastomer sleeve is then snuggly fitted to an outside surface of the wellbore tubular by relaxing the elastically stretching force. The thus prepared wellbore tubular may be lowered into a wellbore in the Earth, and cement may be pumped on the outside of the wellbore tubular to form a cement sheath which fully surrounds the elastomer sleeve.