What was found
WHILE C/W SB 977, CORROSION WAS FOUND ON TOP OF LEFT, AFT WING ATTACH AND LOWER CABIN DOOR POST ATTACH. SUSPECT WATER LEAKING IN AFT CABIN DOOR AT SOME PREVIOUS TIME. LEFT CABIN INTERIOR WAS REMOVED FOR AVIONICS INSTALL AND PROVIDED ACCESS FOR BETTER INSPECTION THAN CALLED OUT IN SB. IN THIS AREA, STEEL SPAR PLATE HOOKS TO ALUMINUM STRUCTURE. MAINTENANCE WAS ABLE TO CLEAN CORROSION AND EPOXY PRIMER WITHOUT REPLACING PARTS. SUBMITTER SUGGESTED INSPECTION OF AREA WHEN C/W S977 (PIPER).