Should I strip off this stuff on the fender?

[lone73]

There is some gray bondo or something on the side of the fender and rear quarter panel.

A few more pics here...
http://home.austin.rr.com/davidsvette/vette/corvette.htm
(Linked)

 

[black_81_vette]

That would depend on the surface underneath .no cracks you should be ok.

 

[Mac]

That is the bonding strip. Fenders are not formed in a single piece; they're molded together and a bonding strip is applied over the junction. I wouldn't sand it off.

-Mac