Abstract: A system for coating a welded joint in butt-welded pipes, comprising the following stages: feeding a first component constituted by a mixture of modified polyurethane resins premixed with a high-density inert material of well defined particle size to the hopper of a screw feeder by way of a regulator valve which meters its throughput; conveying said first component in the form of a slurry by means of said screw feeder into a blade mixer where it is carefully mixed with a second component constituted by a liquid hardening catalyst in a suitable mixing proportion by means of a metering pump and by way of a heat exchanger; and finally filling a form surrounding said joint with the catalyzed mixture leaving said blade mixer.