What was found
ON AN OVERNIGHT, MECHANIC NOTICED A BULGE ON EXTERNAL SKIN AT FRAME 21 LOWER RIGHT SIDE. UPON FURTHER INSPECTION, FOUND STRINGER 21R HAD CORROSION BETWEEN FRAME 21 AND FRAME 22 UNDER TCAS ANTENNA INTERNAL DOUBLER. CUT OUT CORRODED SECTION OF FUSELAGE SKIN AT FRAME 21 AND INSTALLED FILTER AND DOUBLER IN ACCORDANCE WITH EMB120 SRM 53-00-01, PG 219 AND 222. CUT OUT CORRODED SECTION OF STRINGER 21R BETWEEN FRAMES 21 AND 22 AND SPLICED A SERVICEABLE SECTION OF STRINGER IN ACCORDANCE WITH AERODESIGN REPORT 3759-1 REV IR DATED 8/11/00.