AIR TAXI ACFT LANDS ON CLOSED RWY AT IOW DUE TO POOR 'CLOSED RWY' MARKINGS.

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I CANCELED MY IFR ABOUT 5 MI E OF IOW. AFTER LEAVING CEDAR RAPIDS APCH; I CALLED UNICOM FOR AN ARPT ADVISORY AND RECEIVED NO RESPONSE. THE ASOS WAS RPTING WINDS 190 DEGS AT 5 KTS. I ELECTED TO LAND ON RWY 17. I ENTERED A DOWNWIND ON ABOUT A 45 DEG ANGLE. I RPTED ON UNICOM MY DOWNWIND; BASE; AND FINAL. ON FINAL FOR RWY 17 I NOTICED NOTHING UNUSUAL ABOUT THE RWY; NO 'X' WAS VISIBLE FROM THE AIR. JUST BEFORE TOUCHDOWN AS I FLEW OVER THE NUMBERS I NOTICED A SMALL 'X' BTWN THE NUMBERS. AT FIRST IT LOOKED LIKE DEBRIS; THEN I REALIZED THE RWY WAS CLOSED. THIS WAS ABOUT THE TIME I TOUCHED DOWN. THE RWY WAS CLR WITH NO CONSTRUCTION ON IT. THE RWY WAS CLOSED FOR CONSTRUCTION OF A DRAINAGE SYS NEAR THE APCH END OF RWY 35. AFTER TAXIING IN AND GOING INTO THE TERMINAL; THE ARPT MGR MET ME AT THE DOOR WANTING MY CERTIFICATE NUMBER AND NAME TO TURN ME INTO THE LCL FSDO. I APOLOGIZED TO HIM AND TOLD HIM THE X'S ON THE RWY WERE NOT VISIBLE FROM THE AIR. HE WOULD NOT LISTEN AND TOLD ME I SHOULD HAVE CHKED THE NOTAMS. I TOLD HIM I RECEIVED MY BRIEFING OVER DUATS AND THE NOTAM WAS NOT POSTED THERE. I ASKED HIM WHY THE CLOSURE WAS NOT POSTED ON THE ASOS AND HIS REPLY WAS HE DOES NOT PUT NOTAMS ON THE ASOS. AFTER DEPARTING IOW I MADE A CIRCLE OVER THE ARPT AND VERIFIED THAT THE X'S OVER THE NUMBERS COULD NOT BE SEEN FROM THE AIR. THE X'S WERE MADE FROM CONSTRUCTION FENCING WITH SQUARE HOLES IN IT. YOU COULD SEE RIGHT THROUGH THE HOLES. I THINK THE MARKINGS TO CLOSE A RWY SHOULD BE OF A STANDARD SIZE AND COLOR; NOT JUST ANY SCRAP THAT CAN BE FOUND. IN RETROSPECT A GAR WOULD HAVE BEEN THE PROPER PROC; BUT IT WAS JUST A SPLIT-SECOND FROM THE TIME I REALIZED THE RWY WAS CLOSED AND I TOUCHED DOWN.