A LOW TIME C152 PLT; FLYING IN THE VICINITY OF BFI; WAS LOOKING DOWN AT THE GND WHEN HE FINALLY NOTICED A C172; ONLY 30 FT AWAY; MAKING AN EVASIVE TURN.

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I WAS LOOKING AT THE GND TO VERIFY MY POS. I LOOKED UP AND SAW A RED AND WHITE CESSNA ABOUT 60 FT AWAY AND CLOSING; HEADING SBOUND; AND EVASIVELY ROLLING INTO A STEEP BANK AWAY FROM ME TO THE W. I EVASIVELY ROLLED INTO A DIVING TURN TOWARD THE N. IF BOTH THE OTHER PLT AND I HAD NOT TURNED AWAY FROM EACH OTHER; WE CERTAINLY WOULD HAVE COLLIDED. BEFORE THE OCCURRENCE; I HAD RECENTLY MADE INITIAL CONTACT WITH BOEING TWR AND HAD JUST ENTERED BOEING CLASS D AIRSPACE. I WAS NEVER GIVEN A TA BY BOEING TWR. I BELIEVE THAT THE OTHER ACFT WAS FLYING TO RENTON ARPT; WHICH IS LIKELY WHY BOEING TWR WAS NOT AWARE OF IT. I HAVE EXPERIENCED THAT VFR TFC BTWN BOEING TWR AND RENTON TWR OFTEN CONFLICTS. I WOULD RECOMMEND A SYS THAT WOULD ALLOW BOEING AND RENTON TWRS TO BE AWARE OF EACH OTHER'S TFC. IF I HAD RECEIVED A TFC RPT; I BELIEVE THAT ADEQUATE SEPARATION WOULD HAVE BEEN MAINTAINED. I WOULD ALSO RECOMMEND THAT IF BOEING TWR'S SUPPLEMENTAL RADAR; FED FROM SEATAC; HAD A CONFLICT ALERT SYS; IT WOULD HAVE NOTIFIED THE CTLR OF THE SIT.