A method for exposing a substrate to water under superatmospheric pressure at a temperature of at least 70° C. includes (a) applying a reaction mixture to a substrate, which reaction mixture has an isocyanate index of at least 10 and includes an aromatic polyisocyanate component, a polyol component having a polyol with a hydroxyl equivalent weight of at least 500, and a catalyst component having an isocyanate trimerization catalyst, and at least partially curing the reaction mixture to form a polyisocyanurate or polyurethane-isocyanurate polymer having a glass transition temperature of at least 80° C., and (b) exposing the substrate and the polyisocyanurate or polyurethane-isocyanurate polymer to water under superatmospheric pressure at a temperature of at least 70° C.