Abstract: A sealant composition especially useful for stopping punctures in vehicle tires includes: (a) a viscous fluid such as propylene glycol or polypropylene glycol; (b) one or more fibrous materials such as cellulose, wool, nylon, rayon, hair, wollastonite, rock-wool, glass wool fibers, polyester, polypropylene, or other material of like nature; and (c) flat plate-like particles, such as mica. In one embodiment, the composition is too viscous to be readily passed through a pneumatic tire valve stem, so the viscous product is installed into the tire while the tire is dismounted from the wheel. In another embodiment, the composition is made of two parts, each of which is sufficiently low in viscosity to pass readily through the valve stem, but which react on mixing inside the tire form a liquid of higher viscosity.