FO OF A DH8 FAILED TO FOLLOW FLT PLAN TRACK DURING DEP RESULTING IN ATC INTERVENTION AND VECTORS TO INTERCEPT PROPER COURSE AND TO PROVIDE STANDARD SEPARATION WITH INBOUND TFC'S AIRSPACE IN WHICH HE HAD PENETRATED.

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WE ACQUIRED A NEW (TO US) ACFT IN SEATTLE. ONCE WE LOADED PAX AND STARTED ENGS; THE ACFT STARTED PRESSURIZING ON THE GND. THIS WAS BECAUSE MAINT PERSONNEL HAD WORKED ON IT EARLIER AND HAD NOT SET ALL THE PRESSURIZATION CTLS BACK CORRECTLY. THE CAPT MISSED IT ON HIS ORIGINATING FLOW PARTIALLY BECAUSE A SWITCH WAS BENT WHICH MADE IT APPEAR AS THOUGH IT WAS IN THE CORRECT POS. ANYWAY; WE CALLED MAINT AND THEY CAME AND SET THAT SWITCH CORRECTLY. HOWEVER; ONCE WE DEPARTED SEATTLE; WE COULD NOT PRESSURIZE. IN THE COURSE OF DEPRESSURIZING THE ACFT ON THE GND; WE HAD OPENED AN OUTFLOW VALVE THAT REMAINED OPEN BECAUSE WE DID NOT REDO THE ORIGINATING CHKLIST AFTER MAINT LEFT. THAT WAS A DEFINITE MISTAKE. ONCE ENRTE; THE CAPT WAS FUMBLING AROUND TRYING TO PRESSURIZE THE CABIN SLOWLY ONCE I DISCOVERED THE OPEN OUTFLOW VALVE. I WAS FRUSTRATED BY THE CABIN RATE SPIKES AND SO I STARTED HELPING HIM. IN THE PROCESS (IT WAS NOISY BECAUSE OF THE OPEN OUTFLOW VALVE BY OUR FEET); I WAS SUPPOSED TO JOIN THE AIRWAY; BUT EITHER DIDN'T HEAR THE CLRNC; OR FORGOT ABOUT IT. THE END RESULT WAS WE FLEW INTO THE ARR CORRIDOR AND THEY HAD TO VECTOR US OUT. THE CTLR WAS MAD AND I WAS VERY UPSET I GOT FIXATED ON THE ACFT AND THE CAPT'S LACK OF UNDERSTANDING AND DIDN'T TAKE CARE OF PRIORITY #1; TO FLY THE AIRPLANE.