PLT OF WW24 BELIEVES RWY INCURSION AT SPI WAS DUE TO SIGNAGE ISSUES AND FADED TXWY MARKINGS.

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A ROUTINE FERRY FLT WAS FLOWN TO SPI TO DELIVER A WESTWIND II FOR SCHEDULED MAINT. A VISUAL LNDG WAS MADE ON RWY 22 AND THE ACFT TURNED LEFT OFF THE RWY ONTO TXWY H. GND CTL WAS CONTACTED ON 121.9 AND WE WERE CLRED TO; 'TURN LEFT ON TXWY A; CROSS RWY 18; AND HOLD SHORT OF RWY 31.' THE COPLT READ BACK THE CLRNC AS 'HOLD SHORT OF RWY 18;' AFTER WHICH GND CTL RESTATED THE CORRECT CLRNC AS 'HOLD SHORT OF RWY 31.' THIS CLRNC WAS READ BACK CORRECTLY AND UNDERSTOOD BY THE FLT CREW. SHORTLY THEREAFTER GND CTL REPEATED 'HOLD SHORT OF RWY 31.' THEN CLRED US TO 'CONTINUE ON TXWY ALPHA; L TURN ON E; HOLD SHORT OF RWY 22.' THIS CLRNC WAS ACKNOWLEDGED AND COMPLIED WITH. WHILE HOLDING SHORT OF RWY 22; THE GND CTL ASKED US TO CALL THE TWR BY PHONE TO DISCUSS A POSSIBLE PLT DEVIATION. AFTER PARKING THE ACFT; I CALLED THE TWR. THE TWR SUPVR INFORMED ME THAT WE HAD INADVERTENTLY ENTERED AND CROSSED RWY 31 IN VIOLATION OF A CLRNC CAUSING A LIGHT PLANE APCHING THE ARPT TO BE INSTRUCTED TO GAR. HE ALSO INFORMED ME THAT HE WAS ISSUING A RPT ON THE INCIDENT. WE SUBSEQUENTLY REVIEWED THE TAPES THAT AFTERNOON WHICH TO ME SEEMED INCONCLUSIVE. CONTRIBUTING FACTORS: 1) THERE WERE NO STANDARD RED RWY WARNING SIGNS ON TXWY A FOR EITHER RWY 18 OR RWY 31; EVEN THOUGH THE STANDARD RED RWY WARNING SIGNS WERE CLRLY VISIBLE FOR TXWYS INTERSECTING WITH RWY 04-22. 2) ANY PAINTED STRIPES ON THE SURFACE OF TXWY A WERE INDISTINCT AND FADED MAKING IT DIFFICULT TO IDENTIFY THE RWY INTXN; ESPECIALLY FROM THE LOW SEATING POSITION OF A WESTWIND II ACFT. ADDITIONAL COMMENTS: A PLT WHO IS CLRED TO HOLD SHORT OF A RWY SHOULD HAVE A REASONABLE EXPECTATION THAT STANDARD RED RWY INTXN WARNING SIGNS ARE INSTALLED AT ALL RWY CROSSING INTXNS ESPECIALLY AFTER OBSERVING THESE SIGNS AT INTXNS WITH ANOTHER RWY ON THE SAME ARPT. IN THE ABOVE CASE; I WAS SPECIFICALLY LOOKING FOR THESE STANDARD SIGNS TO IDENTIFY THE INTXN WITH RWY 31 SO WE COULD HOLD SHORT OF IT. I FEEL THAT IF THESE RWY WARNING SIGNS ARE NOT CONSISTENT OVER THE ENTIRE ARPT THAT THIS FACT SHOULD BE PUBLISHED AS A NOTAM TO ALERT PLTS TO THIS HAZARD.