EMPLOYEE WAS ACTING AS SPOTTER OUTSIDE TANK ON CREW ASSIGNED TO PRESSURE-WASH TANK INTERIOR. WHILE GATHERING UP TOOLS & EQUIPMENT AT END OF SHIFT, EMPLOYEE WAS ROLLING UP HOSE & HAD BARE HAND ON END OF SPRAY WAND WHEN HE ACCIDENTALLY BUMPED THE WAND TRIGGER - PRESSURIZED WATER CAUSED PUNCTURE WOUND TO LEFT HAND.