My 1988 235/320 has a ventral fin starting just below the TE of the wing to 10" of height at the rudder.   I had a 7" vertical crack in the paint on both sides of the fin just below the point of minimum girth on the fusilage (a point just below  the FE of the horiz Stabilizer)  The cracks did not go into the fusilage. Careful sanding revealed only the paint was cracked, not the fin or fusilage. Put a 2 bid application on either side 6"for and aft of this area and will monitor.  THe paint Sterling Polyurethanc is very brittle in my opinion, but there was some type movement in the tail assy.  Perhaps the larger rudder and fin contributed to the cracks.. I'm considering the  addition of a Horizontal Prepreg bulkhead thru this area.. Something to look for   Regards.   Don SKeele  (850) 626-4946