Abstract: A method and composition for providing gas-impermeable, adherent, flexible, and fire retardant coatings to rock and the like surfaces whereby the rock is supported by the caisson effect and mechanical strength properties of the coatings. The method comprises spraying a two-component polymer such as polyurethane onto a rock surface and spraying a layer of inorganic particulate material such as unexfoliated vermiculite onto the said coating of polymer with or without concurrent application of further layers of said polymer before completion of curing of the coating whereby the particulate material adheres to the polymer. An aqueous dispersion of inorganic particulate material such as vermiculite which may be in the exfoliated form can be applied onto the coating.