What is the closest type of undercoating that GM originally used inside the wheel well areas?

Years ago, I just used some generic auto parts store spray can undercoating and it eventually formed cracks all over where I sprayed it. This time, I want to do it right. I know I will have to strip all the old under coating off, including the original GM stuff (something I didn’t do the last time I did it) and also replace the old GM seam sealer caulking that is cracked or missing in places. The original GM undercoating seems to be very hard and thin so that’s what I want to duplicate this time around.

 

3M Body Shutz is very close to the original stuff.
https://www.3m.com/3M/en_US/p/d/b40071599/

Will also need the 3M siphon feed gun
https://www.3m.com/3M/en_US/p/d/b40069354/

For seam sealer, 3M urethane is good stuff. Much better than what GM used 35 years ago.

 

Thanks.

 

FYI - Finally had my '84 Trans AM, an unrestored original car, up on the hoist yesterday to replace the gear shifter shaft seal. That is another story about cheap Chinese parts. The car had been undersprayed, by whom I do not know, but there not a spec of rust anywhere. It was probably by the dealer as I cannot remember any new vehicle that I or my dad ever purchased came from the factory with undespray. The product easily washes off with lacquer thinner. I used it on the engine oil pan & trans pan.

 

The only factory undercoating was inside the wheel wells and under the quarter panels behind the rear wheels. The undercoating was a textured black hard rubber rock guard type stuff. Any additional undercoating would have been either a dealer or other aftermarket add on.