As I mentioned in one of the other threads, my M20K 252 is at Houston Tank Specialists for a strip and reseal of the right tank.  We noticed some fuel leaking in the area of the lower spar inboard, and it was actually leaking into the cabin along the front of the lower spar.  After opening up the tanks, stripping and cleaning, an area of corrosion was discovered. I only have the one picture of it here, but according to Carl, it's definitely not the surface filiform type that Allan @Hyett6420 had