I used a PPG (shopline) urethane primer on my SMC body, I sprayed the primer on SMC that was sanded 80 grit for mechanical "grip" - no issues here :1st:
If I had to do it again I'd use PPG DP epoxy primer, that stuff seems to stick to almost anything. The problem with epoxy is that you only have a few days window to topcoat it. If you wait too long you have to sand it and spray another coat epoxy.
For a sheetmetal body I'd definitely use epoxy for the first coat over bare metal.
I also used the epoxy primer/sealer from tcpglobal (Kustomshop), gallon incl activator for $100, half the price of PPG and seems to be pretty good also....