inglés al español Técnico/Ingeniería Construcción / Ingeniería civil

Can somebody help me with the "flux" translation? The context is about replacing shut off valves at home. The text is as follow: "Choose the fixture where you want to install a shut off valve; shut off the main water supply; mark and cut the pipe with a pipe cutter; sand both ends of the pipe and FLUX them; sand and FLUX the inside fittings of the valve". I put the capital letters.
I have been looking up the flux meaning, but it only appears as a name; in this case is obviously a verb. My try: "Escoge el artefacto donde quieres instalar una llave de paso. Cierra el suministro principal de agua y, con una cortadora de tubos, marca y corta el tubo. Lija ambos extremos del tubo y úntale un fundente; haz lo mismo para el interior de la llave".