Abstract: The foam application apparatus or gun of this invention is particularly adapted to prevent leakage of the gun through the nozzle while assuring repeated use without seizure. The foam application apparatus includes a cylindrical mixing chamber having two inlet passages and an outlet passage through which mixed foamable resin is sprayed or discharged under pressure. A cylindrical valve rod having a free end is reciprocated in the mixing chamber from an "off" position, wherein the rod free end blocks the communication from the inlet passages to the outlet passage, and "on " position, wherein the rod is retracted, providing unrestricted communication between the inlet and outlet passages. In one preferred embodiment, a pair of closely spaced radial grooves are provided on the free end of the valve rod. The grooves receive fluid resin to form in situ resin seals which are replenished each time the rod is reciprocated in the chamber.