ACR X TAXIWAY EXCURSION. PLTDEV. SITUATION: POOR TAXIWAY MARKING SIGNS.

Narrative

ACR X UPON LEAVING THE GATE AT SFO WE WERE ASSIGNED RWY 10R FOR DEP. HAVING NEVER USED THIS RWY FOR TKOF; THIS SECTION OF THE ARPT WAS UNFAMILIAR; COMBINED WITH THE FACTS THAT IT WAS A DEP AND IT WAS RAINING. TAXIING ALONG THE INNER TAXIWAY A UNTIL S WE APCHED THE DEP END OF RWY 10R. DUE TO THE TIME AND THAT THERE WAS NO TFC; TWR CLRED US FOR TKOF PRIOR TO APCHING THE HOLD SHORT. SO WHILE TRYING TO LOOK OUTSIDE; IN THE RAIN; AT NIGHT; AT CONFUSING TAXI LIGHTS; AND AT THE ARPT DIAGRAM; WE TAXIED ONTO RWY 10R AND DEPARTED. NO FURTHER INCIDENT OCCURRED. LOOKING BACK AND STUDYING THE ARPT DIAGRAM VERY CAREFULLY WE HAD TURNED R OFF OF TAXIWAY R ONTO RWY 10R AND CROSSED A SMALL SECTION OF PAVEMENT WHICH WAS NEITHER RWY NOR TAXIWAY. AT THE TIME DURING THE RAIN; DARKNESS; CONFUSING TAXIWAY LIGHTS AND RWY IDENT SIGNS; WE APPEARED TO HAVE TAXIED CORRECTLY. ONLY UPON AN UNCERTAIN FEELING AND A SECOND LOOK; WE FIGURED OUT EXACTLY WHAT HAD HAPPENED. NOTHING WAS DAMAGED AND WE WERE LUCKY BUT NEXT TIME IF YOU ARE UNSURE; STOP AND ASK.