We have had to repair about ten of the rear spars on the 1979 and on Cheetahs and Tigers. From 1979 and on Grumman changed the bent sheet metal spars to an extruded spar. The extrusions are far more likely to develop Intergranular corrosion almost exclusively at the flap rub strip. The problem is very prevalent with the AG5B built in the 90’s. It’s very rare to find this problem on the Savannah built airplanes. If there is even the slightest sign of corrosion at the rub strip. Remove the strip, clean and paint before it ruins the spar section. We have a DER approved repair that we perform in our shop.