B757-200 REQUIRES DEICING DUE TO SPRAY FROM FIRE TRUCKS.

Narrative

WE HAD JUST FLOWN INTO SLC. OUR DEP WX WAS SUNNY SKIES. NOTHING TO CAUSE ANY ICING ON AN ACFT. AT OUR DEP GATE; WE MET THE FO COMING OFF THE INBOUND FLT. HE COMMENTED THAT THE CAPT MIGHT BE A LITTLE SLOW GETTING OFF THE ACFT. HE WAS SAYING GOOD-BYE TO EVERYONE BECAUSE IT WAS HIS 'RETIREMENT FLT.' THE EXTERIOR WALKAROUND INSPECTION WAS NORMAL -- NOTHING UNUSUAL NOTICED. JUST PRIOR TO PUSHBACK; WE RECEIVED THE FUEL SLIP. THE ONLY THING UNUSUAL ABOUT IT WAS THE ACFT CAME INTO SLC WITH ALMOST FULL WING FUEL TANKS. JUST AFTER PUSHBACK; A FLT ATTENDANT INFORMED US THERE COULD BE ICE ON TOP OF THE WINGS I SENT MY FO BACK INTO THE CABIN TO CHK; AND SURPRISINGLY; WE DID HAVE PATCHES OF ICE ON TOP OF THE WINGS (NOT VISIBLE FROM THE GND). WE CALLED FOR DEICING; DEICED THE ACFT; AND DEPARTED FOR ORD. THE REMAINDER OF THE FLT WAS UNEVENTFUL. IT WAS A COOL DAY (43 DEGS F). WATER HAD BEEN SPRAYED ON THE ACFT BY FIRE TRUCKS AS IT TAXIED INBOUND TO SLC (A SALUTE TO THE PREVIOUS CAPT'S RETIREMENT); AND THE WING FUEL TANKS WERE ALMOST FULL FROM THE PREVIOUS FLT (WITH SUPER-COOLED FUEL NEXT TO THE SKIN). ALL 3 OF THESE EVENTS HAD TO OCCUR SIMULTANEOUSLY FOR ICING TO EVEN FORM. THE LAST 2 EVENTS WERE VERY UNUSUAL. WITH THE WX CONDITIONS WE HAD; ONE WOULD NOT EXPECT ANY ACFT ICING; BUT ADD IN THE OTHER UNUSUAL CONDITIONS AND ICING CAN (AND DID) OCCUR.