The invention concerns a method of applying a coating in a liquid state to wellbore tubing, such as threaded connector parts or casing sections, before running the tubing downhole. The coating only cures while running downhole. The coating is an elastic curable coating composition, comprising a curable elastic thermosetting resin, a curing agent and a water-absorbing expansion agent. During make-up of a connector, the liquid coating composition acts as lubricant. When cured, the coating will swell when contacted with a wellbore fluid to ensure leak-tightness.