A CESSNA 206 PLT WAS VFR IN IMC NEAR DSD.

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ENRTE FROM S21 TO UAO; VFR; I REQUESTED AN IFR CLRNC. I RECEIVED A SQUAWK CODE AND WAS ASKED; DID I WANT THE MEA OR STAY AT MY CURRENT ALT OF 8500 FT. I STAYED AT 8500 FT BECAUSE THE MEA WAS 12500 FT AND I HAD AT LEAST 2000 FT TERRAIN CLRNC. THE CTLR SAID I WOULD RECEIVE LOWER IN 13 NM. I WAS VFR AT THIS TIME AND REMAINED VFR FOR 20 NM. AT THE TIME I WAS GOING TO REQUEST LOWER; I GOT HANDED OFF. AT THE TIME CONTACT WAS MADE WITH THE NEXT CTLR; I HAD ENTERED IMC. AT THE HDOF; I TOLD THE CTLR 'WITH YOU AT 8500 FT.' I DID NOT SAY IFR TO UAO; WHICH I SHOULD HAVE DONE. THE CTLR ACKNOWLEDGED WITH AN ALTIMETER SETTING BUT DID NOT STATE 'MAINTAIN VFR.' I WAS IMC FOR 20 NM (10 MINS AT 140 KTS). I WAS NOW VFR BTWN LAYERS. AT THE NEXT HDOF THE CTLR CLRLY STATED 'MAINTAIN VFR.' I WAS ABLE TO DO SO. ON MY FINAL HDOF; I REQUESTED THE LOC APCH INTO UAO. THE CTLR CLRLY STATED 'MAINTAIN VFR.' ABOUT 2 MINS LATER; THE CTLR CLRED ME TO 6000 FT AND I WAS INFORMED I WAS #2 FOR THE APCH; AND I WAS GIVEN VECTORS FROM THAT POINT UP TILL THE APCH. THE APCH WAS SUCCESSFUL WITH A SAFE LNDG. I DIDN'T WANT TO FLY AT 12500 FT BECAUSE OF HYPOXIA. MY PERSONAL LIMIT IS 10000 FT. BECAUSE I WAS TOLD I WOULD RECEIVE LOWER IN 13 NM AND I COULD SEE AT LEAST 20 NM; I DIDN'T CONSIDER IT A PROB. I SHOULD HAVE TIMED THE 13 NM AND REQUESTED AT 10 NM AFTER INITIAL CONTACT. HAD THE SECOND CTLR STATED 'MAINTAIN VFR' THIS WOULD HAVE GIVEN ME A HEADS UP. HOWEVER; I ALSO SHOULD HAVE STATED IFR TO UAO. THE CONFUSING POINT WAS WITH THE LAST CTLR. HE DID NOT STATE CLRED TO UAO VIA VECTORS OR ANY STATEMENT OF IFR CLRED; JUST ALT AND HDG. I DID WONDER WHY THE SECOND CTLR DIDN'T QUERY MY VFR ALT. I SHOULD HAVE QUERIED HIM. IT IS CLR THAT A PLT SHOULD NEVER ASSUME ANYTHING.