What was found
AIRCRAFT WAS IN MAINTENANCE FOR A ROUTINE 200 HOUR INSPECTION. MAINTENANCE WAS COMPLYING WITH MULTI-ENGINE OWNER ADVISORY MEL-57-02 WING- FORWARD LOWER CARRY-THRU SPAR CAP INSPECTION. DURING THIS INSPECTION CORROSION WAS NOTED ON THE CARRY-THRU SPAR LOWER FORWARD LEFT FITTING. REFERENCE TO SID 57-10-10 #4 INSPECTION INSTRUCTIONS ITEM C INSPECT THE SPAR AND FITTING FOR CORROSION. IF CORROSION IS FOUND ON THE FITTING INSTALL A NEW FITTING. WE CONTACTED CESSNA STRUCTURES FOR REPAIR PROCEDURES AND WAS TOLD "PER ENGINEERING, ONCE THE SPAR FITTING GETS DAMAGED (WHETHER GOUGING OR CORROSION) IT MUST BE REPLACED". "PART REPLACEMENT SHOULD BE DONE PER BEST SHOP PRACTICES".