I just posted to this thread
http://forums.maxperformanceinc.com/...d.php?t=696852 yesterday. I had the same question about high build primer. Details and pics in the thread.
To follow up, I felt I needed to straighten out the lower rail a bit more after I'd primed it. I actually laid a block of wood on the part I wanted to bend and hit it with a sledge hammer. It put two indentations in the bumper where the wood was in contact but NO cracks in the primer.
The two indentations were completely gone after about 3 hours, just as Pontiac said back in the day....but no cracks in the epoxy.
I was using Shopline JP-375 which is the 'cheap stuff' but I've had great results using it and it actually sands quite well.