AN M20 SINGLE PLT; FLYING VFR; WAS UNABLE TO COMPLY WITH THE ALT REQUEST MADE BY I90.

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FLYING A MOONEY M20E AT APPROX 7500 FT EBOUND; I APCHED THE HOUSTON CLASS B AIRSPACE FROM IDU ON RTE TO IWS AT APPROX XA45 CDT. THERE WAS A SCATTERED LAYER BELOW ME WITH CEILING APPROX 4000 FT AND TOPS AT APPROX 6000 FT. IT WAS APPARENT THAT THE UNDERCAST WAS DENSER TOWARDS THE W; INCREASING TO A BROKEN; IF NOT UNDERCAST LAYER. I CONTACTED HOUSTON APCH AND REQUESTED CLRNC TO TRANSIT THEIR AIRSPACE ON RTE TO IWS. I EXPLAINED THAT THERE WAS AN UNDERCAST LAYER; AND THAT I WOULD LIKE TO STAY ON TOP UNTIL I WAS CLOSER TO IWS. I EXPLAINED THAT THE UNDERCAST LAYER LOOKED DENSER TO THE W. APCH ISSUED A XPONDER CODE AND CLRED ME INTO THE CLASS B AIRSPACE. APCH CLRED ME TO DSND TO THE S. I TOLD APCH THAT CLOUD COVERAGE WAS DENSER TO THE S; AND THAT N WOULD BE PREFERABLE. APCH INSISTED THAT I DSND TO THE S. I FOUND AN AREA OF ADEQUATE CLOUD CLRNC TO MAINTAIN VFR MINIMUMS FOR A DSCNT; AND BEGAN MY DSCNT TO THE S. AS I DSNDED; BTWN THE CLOUDS; I INITIATED TURNS TO MAINTAIN REQUIRED VFR CLOUD CLRNC MINIMUMS. AS I DSNDED THROUGH APPROX 5000 FT; APCH ISSUED THE STATEMENT 'MAINTAIN 4500 FT.' I IMMEDIATELY ADVISED THAT I WAS UNABLE TO MAINTAIN 4500 FT DUE TO CLOUDS; AND REQUESTED LOWER ALT. AT MY POS; I HAD CLOUDS IN FRONT OF ME; AND CLOUDS ON EITHER SIDE AND BEHIND. I COULD NOT CLB FAST ENOUGH TO GET ABOVE THE CLOUDS; AND FLYING STRAIGHT OR DEVIATING IN EITHER DIRECTION WOULD HAVE PUT ME IN IMC WITHIN SECONDS. APCH DID NOT RESPOND IMMEDIATELY. APCH ISSUED A TFC NOTIFICATION TO ME AND ANOTHER ACFT. NO OTHER CONFLICTING TFC WAS VISIBLE. IN ORDER TO MAINTAIN VFR MINIMUMS; AND TO REMAIN OUT OF IMC; I CONTINUED MY DSCNT TO APPROX 3500 FT. APCH STATED THAT I HAD NOT COMPLIED WITH THE ALT ASSIGNMENT. I ADVISED AGAIN THAT I WAS UNABLE; AND THAT I HAD REQUESTED LOWER. APCH THEN REQUESTED THAT I TELEPHONE UPON LNDG. UPON LNDG; I TELEPHONED APCH AND EXPLAINED THE CLOUD COVERAGE SIT AND THE NECESSITY OF DSNDING IN ORDER TO MAINTAIN VFR MINIMUMS AND STAY OUT OF IMC.