With credit for this pic stolen from a post by "Clay C"...

After his sandblasting we can clearly see the tan-colored seam sealer.  But what, exactly, is this black stuff?  Did Nissan use panel bond on the Z?

 

I have also noticed seams on the forward surfaces of the rear inner wheel wells that have a "bead" of some material that looks like it oozed out from between the panels.  It is hard (unlike seam sealer) and I've wondered if it was panel bond.

 

 

 It’s the black goo that that leaks out and spoils the weld when you’re installing over fender flairs and have cut out the wheel well lip, then weld the two layers back together, or try to at least.  

It’s a caulking, not a bonding agent. Maybe some fear of water getting in the air vent holes and getting down in that area.  

8 hours ago, zKars said:

 It’s the black goo that that leaks out and spoils the weld when you’re installing over fender flairs and have cut out the wheel well lip, then weld the two layers back together, or try to at least. 

Amen, I'm sure it felt like a really good solution to a problem area back in the day but what it really did, at least in the Z's I've worked on is hold water right at the edge of the lip.

@Broman, the seam at the top that you're referring to, the black goo did its job well.