LOW TIME GA PLT ENCOUNTERS ICING ON ASSIGNED RTE; COMPLAINS THAT HIS FILED ROUTING WAS NOT GRANTED. HE CHANGED ALT WITHOUT ADVISING ATC OR DECLARING AN EMER.

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DEPARTING MHT; CALLED FSS AND WAS ADVISED OF RPTED ICING N OF A LINE FROM V106 TO BARNES W TO LAKE HENRY. RPTED AT 6000 FT. DECIDED; SINCE GPS IFR EQUIPPED; AND SINCE UPPER LEVEL WINDS WERE AGAINST ME AT HIGHER ALTS; TO FILE AT 4000 FT; DIRECT PAWLING; DIRECT READING. THIS ACTION AFFORDED ME BOTH 1) THE SAFEST RTE AND 2) BEST ALT FOR PROGRESS AGAINST 35 KT WESTERLY. UPON CALLING MANCHESTER CLRNC; I RECEIVED; TO MY CHAGRIN; THE FOLLOWING ROUTING: MANCHESTER 3 DEP; KEENE; V93; V292; V408; LAKE HENRY AS FILED. I WAS AWARE WITHIN 10 MINS THAT THIS WAS NOT GOOD. WHILE ON CLBOUT THE DEP CTLR REQUESTED A CLB TO 6000 FT. WHILE PASSING THROUGH A THIN LAYER AT 5800 FT; I ENCOUNTERED LIGHT RIME. REQUESTING HIGHER; I RECEIVED 8000 FT; WHICH PUT ME ON TOP BY 1000-1500 FT. WITH A GND SPD OF 75-80 KTS; THE NORMALLY 2 HR JOURNEY WAS GETTING LONGER. DURING THIS FLT; I HEARD ON MORE THAN ONE OCCASION; LAUGHTER FROM CTLRS WHEN IN CONTACT. AT THE POINT OF THE INCIDENT; I WAS FORCED TO GO THROUGH THE TOP OF ONE OF THE CLOUDS; TO MAINTAIN MY ASSIGNED ALT. THE ENCOUNTER LASTED LESS THAN 20 SECONDS. HOWEVER; I ACCUMULATED 2 INCHES ON THE FUEL CAP AND COVERED 50% OF THE WINDSCREEN. IMMEDIATELY AFTER THAT EXIT; ANOTHER LARGER; DENSER LOOKING CLOUD LOOMED WITH PENETRATION IMMINENT. I EXECUTED A CLB OVER THE TOP. ALT AT APEX WAS 8600 FT; AFTER WHICH I NOTIFIED ATC AND REQUESTED A BLOCK ALT TO REMAIN CLR. THIS ENTIRE EPISODE WAS DETRIMENTAL TO SAFETY. THE ROUTING ASSIGNED BY ATC WAS DIRECTLY RESPONSIBLE FOR THE UNSAFE CONDITION. MY FLT 'AS FILED' WOULD HAVE INVADED NO AIRSPACE; WAS OUT OF ANY DANGEROUS CONDITIONS; AND AFFORDED THE BEST AND SAFEST RTE TO DEST. IN OUR OPINION; ATC FLAUNTED AUTH AND REFUSED THE 'AS FILED' SIMPLY BECAUSE IT WAS GA. THIS IS WHY SO MANY FLY VFR -- I MAY JOIN THEM!