a method and apparatus for isolating a portion of a powder delivery system from contamination includes an anti - contamination valve interposed between the discharge outlet of a sieve in the powder delivery system and the inlet to a powder feeder device communicating with one or more spray guns associated with a powder spray booth . the valve is movable between an open position in which powder discharged from the sieve is permitted to flow unobstructed to the powder feeder device , and a closed position which effectively seals or isolates that portion of the powder delivery system downstream from the sieve .