AN AC90 ON TKOF CLB BTWN 700-1000 FT RETURNED TO THE DEP ARPT DUE TO CAPT'S ALTIMETER READING FIELD ELEVATION.

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HAD TO HAVE ACFT DEICED PRIOR TO FIRST TKOF OF THE DAY. TKOF NORMAL. AFTER CLEAN-UP ON CLBOUT; TWR ASKS 'WHAT'S YOUR ALT?' I HAD JUST GONE FROM OUTSIDE TO INSIDE VISUAL CUES. ALL OUTSIDE INDICATIONS NORMAL; HOWEVER; CAPT ALTIMETER IS STILL READING FIELD ELEVATION (NO CLB INDICATED). I TOLD TWR THAT ALTIMETER WAS INACCURATE (COPLT'S SIDE LOOKED OK). ALSO TOLD TWR THAT I WANTED TO RETURN FOR LNDG BECAUSE OF THIS PROB -- LNDG NORMAL. ON SHORT FINAL; THE ALTIMETER STARTED WORKING AND THE AIRSPD ON THE CAPT'S SIDE FLUCTUATED A COUPLE OF TIMES; AS IF IT WAS GUSTY OUTSIDE (NO WIND TO SPEAK OF). SHUT DOWN ON RAMP AND INSPECTED PITOT TUBES (BOTH HOT) WITH 5 STATIC PORTS. FOUND VISIBLE MOISTURE IN ONE AND REMOVED IT. MADE TEST FLT AROUND THE PATTERN. ALL NORMAL. PICKED UP PAX AND WENT ON WAY. ALL REMAINED NORMAL. FSDO MAINT INSPECTOR CALLED LATER THAT DAY AND I RELAYED THE ABOVE TO HIM. I QUESTION THE WISDOM OF DEICING WITHOUT MAYBE PLACING TAPE OVER STATIC VENTS. I WILL DO THIS IN FUTURE. THE RPTR STATED THE AIRPLANE WAS OVERNIGHT IN A FREEZING RAIN AND WAS BEING DEICED WITH A MIXTURE OF WATER AND DEICING FLUID AT 170 DEGS. THE RPTR OBSERVED THE DEICING CREW SPRAYING THE FUSELAGE BUT DID NOT CONSIDER THE SPRAY AFFECTING THE STATIC PORTS. ON RETURN TO THE FIELD; ONE AFT STATIC PORT WAS DISCOVERED WITH STICKY DEICER FLUID AND VISIBLE MOISTURE AND THIS WAS CLRED. A TEST FLT PROVED THE ALTIMETER NOW WORKED PROPERLY. THE RPTR NOW CARRIES A ROLL OF TAPE TO COVER THE STATIC PORTS WHEN DEICING IS REQUIRED.