PLT OF A BE36 CONFUSES 2 TRACON CTLRS BY DEVIATING FROM HIS RTE WHILE ON AN IFR FLT PLAN AND REQUESTING A 'VFR ON TOP AT 6500 FT.'

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I DEPARTED FREDERICK ARPT WITH IFR CLRNC DIRECT TO EMI VOR THEN AN AIRWAY RTE OVER MRB VOR AND ON TO A DEST OF UNI. UPON INITIAL CALL TO BWI DEP; I NOTIFIED BWI THAT I WAS IN VFR CONDITIONS AND ASKED FOR VFR ON TOP CLRNC. AT SAME TIME I BEGAN A TURN TO THE W TOWARDS UNI ON MY OWN NAV INSTEAD OF FLYING THE IFR CLRNC RTE AS OUTLINED IN THE AIM. THE CTLR THOUGHT I WISHED TO CANCEL IFR AND PROCEED VFR. I INFORMED HIM THAT I WANTED VFR ON TOP TO DEST AND WAS PROCEEDING VFR ON TOP AT 6500 FT. AT THAT POINT; I HAD GONE FAR ENOUGH W TO CROSS INTO DULLES SECTOR'S AIRSPACE AND WAS TOLD TO CALL DULLES APCH. ON INITIAL CALL-UP; I WAS INFORMED THAT BWI APCH HAD SAID I WAS NOW VFR AND I AGAIN RESTATED I WANTED TO BE IFR WITH VFR ON TOP OPS. HE RESPONDED WITH A 'ROGER' AND I DIDN'T HEAR HIS ENSUING ALT ASSIGNMENT TO MAINTAIN 6000 FT. I CONTINUED THROUGH 6000 FT TO 6500 FT FOR VFR ALT BELIEVING I WAS APPROVED FOR VFR ON TOP OPS. I HAD ESTABLISHED MYSELF DIRECT TO MRB VOR; WHICH WASN'T MY IFR CLRNC RTE. WHEN THROUGH 6300 FT; THE CTLR QUERIED AGAIN ABOUT MY INTENTIONS FOR VFR AND I REQUESTED VFR ON TOP AT 6500 FT. AT THAT POINT; HE SAID THAT CANCELLATION WAS RECEIVED AND TO MAINTAIN VFR CONDITIONS. AFTER FLT FOLLOWING THROUGH REST OF DULLES AIRSPACE AND A HDOF TO ZDC; I REQUESTED AND RECEIVED AN IFR CLRNC FOR REST OF TRIP AT 6000 FT. REST OF TRIP MADE WITHOUT INCIDENT. THIS WAS A VERY CONFUSING COUPLE OF MINS FOR ME AND APPARENTLY THE CTLRS AS WELL. I ERRED IN ASSUMING THAT I COULD OBTAIN A VFR ON TOP CLRNC AND PROCEED DIRECT INSTEAD OF FOLLOWING THE IFR CLRNC I HAD BEEN ISSUED ON THE GND AT FDK BY BWI CLRNC. THE CTLRS; IN MY OPINION; DIDN'T KNOW WHAT VFR ON TOP WAS OR HOW TO ISSUE THE CLRNC TO ME; OR WHAT I WAS EXPECTED TO DO ON A VFR ON TOP OP. PERHAPS IF I WOULD HAVE CONTINUED ON MY CLRNC RTE TO EMI; AND ALLOWED THEM TO SORT OUT WHAT TO ISSUE TO ME; IT ALL WOULD HAVE GONE SMOOTHER. WE NEVER HAD ANY TFC CONFLICTS; OR SAW TFC COME TOO CLOSE; BUT IT WAS A COMS NIGHTMARE. AS A RESULT OF THE OCCURRENCE; I RESEARCHED THE VFR ON TOP PROCS FOR MYSELF AND FOR CTLRS. I AM MORE COMFORTABLE WITH THE PROC; ESPECIALLY WITH THE UNDERSTANDING THAT: 1) IT MUST BE APPROVED BY ATC FIRST; 2) IT ISN'T AUTOMATICALLY A CLRNC TO RESUME OWN DIRECT NAV; AND 3) CTLRS IN THIS AREA OF THE COUNTRY DON'T USE VFR ON TOP PROCS VERY OFTEN AND MAY NOT KNOW WHAT TO DO IF IT IS REQUESTED.