Abstract: A method of sealing a porous heating or cooling jacket of a reactor vessel includes the steps of: (a) draining the heating or cooling fluid from jacket; (b) washing and drying the interior of the jacket; (c) filling the jacket with a curable liquid sealant composition under pressure sufficient to cause the composition to permeate the porous areas in the jacket but insufficient to burst the jacket, the composition being curable at temperatures between about 75.degree. C. and 100.degree. C.; (d) draining the sealant from the jacket; and (e) heating the jacket to or above the cure temperature of the composition and maintaining that temperature for sufficient time to ensure total polymerization of the reactant composition.