MLG WAS FLOWN WITH KNOWN DAMAGE NOT WRITTEN UP IN LOGBOOK.

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DEPARTING AT XAAM ON FLT ORD TO COLUMBUS; OH; WE GAVE THE ACFT A COMPLETE PREFLT. THE F/O DID THE COMPLETE PREFLT AND I DID A DUPLICATE EXTERNAL WALK AROUND WHICH IS MY PRACTICE WHEN TIME PERMITS. WE FOUND ALL NORMAL AND DEPARTED NORMALLY AND ARRIVED NORMALLY AT CMH. WE PREPARED FOR DEP AT CMH WITH THE F/O AGAIN DOING A PREFLT. BUT; SITTING AT THE GATE ABOUT 5 MINS BEFORE DEP; ONE OF THE GND SVC PEOPLE RPTED A HOLE (ABOUT 1/2' WIDE AND ABOUT 1 1/2' LONG) IN THE BOTTOM SKIN OF THE AFT STAIR DOOR ON THE ACFT. THE F/O WENT TO INVESTIGATE AND FOUND THIS SMALL WEDGE SHAPED HOLE AS RPTED; PROBABLY CAUSED BY SOMEONE LOWERING THE STAIRS ON SOMETHING. HE RPTED THERE WAS NO OTHER DAMAGE AND THE ADJACENT STRINGER WAS NOT EVEN BENT. WE DECIDED THAT THE DAMAGE WAS NOT SERIOUS AND HAD NO EFFECT ON THE AIRWORTHINESS OF THE ACFT; SO WE DEPARTED ON TIME FOR CHICAGO. ON ARR AT ORD; WE RPTED THE HOLE TO MAINT AND THEY REPAIRED IT. WE LEARNED AT THAT TIME THAT THE FAA WAS NOT NOW PERMITTING TEMPORARY REPAIR OF SKIN DAMAGE AND A FULL SHEET METAL REPAIR HAD TO BE DONE. WE FURTHER LEARNED THAT ALL SKIN PUNCTURES IN THE ACFT; NO MATTER HOW INSIGNIFICANT; HAD TO BE RPTED TO THE FAA. WE CONSIDERED THIS DAMAGE INSIGNIFICANT AND THOUGHT IT WOULD BE COVERED WITH TAPE UNTIL A FULL REPAIR COULD BE MADE. WE WERE WRONG. SUPPLEMENTAL INFO FROM ACN 88396: THE CAPT ELECTED; AFTER I ADVISED HIM; WE SHOULD WRITE IT UP IN CMH; TO GO ON TO ORD AND HAVE IT LOOKED AT THERE.