RPTR IFR ENCOUNTERED MODERATE MIXED ICING AND HAD TO DSND BELOW RADAR VECTOR ALT TO GET INTO THE CLR AND CANCEL IFR.

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IFR ASSIGNED ALT OF 17000 FT MSL IN VFR CONDITIONS; WITH CLOUD CEILING BELOW AROUND 11000 FT MSL. I COULD SEE A DIFFUSE CLOUD BUILDUP APPROX 20-30 MI DIRECTLY AHEAD ON THE ASSIGNED RTE; BUT IT DID NOT APPEAR TO BE A DEVELOPING CELL BECAUSE OF THE DIFFUSED NATURE. TALKED TO ATC (ZDV) TO DETERMINE WHAT KIND OF RIDES HAD BEEN RPTED THROUGH THIS AREA. ATC INDICATED THAT LAYERS HAD BEEN RPTED. I CONTINUED FLT INTO THE DIFFUSED CLOUDS AND WITHIN A SHORT TIME (1-2 MINS) HAD ENCOUNTERED MODERATE MIXED ICING. I CALLED CTR TO INDICATE I HAD TO DSND. I BEGAN MY DSCNT TO 15000 FT MSL. CTR INDICATED THERE WAS HIGH TERRAIN AND THE MVA WAS 16000 FT MSL. UPON REACHING 15000 FT I HAD VISUAL CONTACT WITH THE GND AND INDICATED TO CTR I WAS DSNDING BELOW THE CLOUD BASE. AFTER ESTABLISHING VFR AT AROUND 12000 FT MSL I CALLED CTR TO CANCEL IFR AND THEN CONTINUED VFR TO DRO. I AM AWARE OF ICING IN WESTERN UNITED STATES AROUND ANY CLOUD BUILDUP AT ALMOST ANY TIME OF YR. HERE IS WHAT I WILL DO IN THIS SIT IN THE FUTURE: 1) IF I AM VFR ON AN IFR FLT PLAN AND I SEE A CLOUD BUILDUP WHICH CAN BE CIRCUMVENTED; I WILL IMMEDIATELY ASK FOR A RTE DEV AND IF THIS CANNOT BE OBTAINED I WILL CANCEL IFR. 2) I WILL AVOID EVEN DIFFUSE (UNDEFINED) BUILDUPS AT ANY HT ABOVE 12000 FT MSL.