SMA RECEIVES SVFR TO DEPART BUT GETS CAUGHT AND ENTERS CLOUDS.

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WX RPT ATIS; 600 FT OVCST 3 MI; FORECAST VFR N AND W. FROM GND; I COULD SEE BLUE SKY THROUGH THIN BROKEN LAYER. REQUESTED SPECIAL VFR OUT OF CTL ZONE AND FLEW THROUGH HOLDS ON TOP OF BROKEN LAYER; GOOD VISIBILITY ABOVE; NEXT LAYER FORMED 1700 MSL ABOVE; CONTINUED BTWN LAYERS AT 11- 1200 FT AGL; GOOD VISIBILITY BTWN LAYERS. DETERIORATED TO MARGINAL VFR MAINTAINING VFR (LESS THAN 1200 FT AGL; CLR CLOUDS; VISIBILITY GREATER THAN 1 MI). APPROX 20 MI W COULD SEE BLUE SKY AND UPPER LAYER BREAKING UP. I CLBED TO GET ON TOP. JINKING AROUND CLOUDS BUT AT ONE POINT GOT CAUGHT IN A 'CLOUD CANYON.' SAFEST THING TO DO AT THAT POINT WAS TO CONTINUE CLB STRAIGHT AHEAD; MOMENTARILY GOING THROUGH A CLOUD. NO MORE THAN A COUPLE OF SECONDS. THEN; I WAS TOTALLY IN CLR; UNDERCAST BROKE UP CONTINUED WITHOUT FURTHER INCIDENT. I WILL NEVER GO SPECIAL VFR/MARGINAL VFR AGAIN -- TOO EASY TO BE TRAPPED. I WAS FOOLED BY GREAT WX LOOKING OVER FIELD WHICH DETERIORATED. ALTHOUGH I WAS LEGAL (EXCEPT FOR PUNCHING THROUGH THAT 1 CLOUD) IT WAS NOT SMART; AN INEXPERIENCED PLT COULD HAVE FOLLOWED ME OUT AND GOTTEN INTO BIG TROUBLE. 1 MI VISIBILITY/CLR OF CLOUDS VFR MAY BE OKAY LEGALLY; BUT SHOULD BE DISCOURAGED -- ESPECIALLY TO INEXPERIENCED PLTS.