ZSE CTLR EXPERIENCED OPERROR AT 2500 FT WHEN THE ISSUANCE OF AN IFR CLRNC TO A MIL ACFT; IN MVFR CONDITIONS; RESULTED IN UNEXPECTED TURNS.

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I SHOULD NOT HAVE BEEN IN SUCH A HURRY TO OFFER IFR CLRNC UNTIL I KNEW THE ACTUAL SPD AND DIRECTION OF ACFT X; EVEN THOUGH I DID IT BECAUSE I BELIEVED ACFT WOULD HAVE TROUBLE MAINTAINING VFR. IF PLT HAD TURNED AT STANDARD RATE OR BETTER; ERROR WOULD NOT HAVE OCCURRED. I WAS WORKING SECTOR 33 WHEN ACFT X (AN AJET) CHKED ON AT XA46:13; VFR; REQUESTING IFR CLRNC TO NZY. I TOLD PLT AT XA46:25 TO SQUAWK XXXX AND MAINTAIN VFR. AT XA46:48 I IDENTED TARGET; TOLD PLT 'RADAR CONTACT' AND ASKED PLT IF HE COULD MAINTAIN VFR UNTIL REACHING 2000 FT (THE MIA IN THAT AREA). THE PLT STATED 'JUST BARELY CAN; UH; DO THAT SIR; YES SIR.' I DID NOT SOUND CONVINCED; SO I ASKED PLT IF HE COULD MAINTAIN HIS OWN TERRAIN AND OBSTRUCTION CLRNC UNTIL REACHING 2000 FT. THE PLT SAID HE COULD; AND I ISSUED HIM A CLRNC TO NZY VIA SEA; THEN AS FILED; AND TO FLY HDG 130 DEGS AND MAINTAIN 2000 FT (AT TIME XA47:03). I THEN OBSERVED TARGET HEADING W; INSTEAD OF SE; AND TOLD PLT TO TURN HDG 070 DEGS (AT TIME XA47:30). AT XA47:54 I ASKED PLT OF ACFT X TO VERIFY HDG 070 DEGS. HE STATED 'IN THE L-HAND TURN NOT THERE YET.' I CALLED TFC FOR ACFT X AT XA48:00 TO TRY TO GET TFC IN SIGHT; AND UTILIZE VISUAL SEPARATION; BUT WAS UNSUCCESSFUL.