Get with your local PPG dealer.I use their DP40/401 epoxy primer all the time.Comes in red oxide, gray, white, graygreen or black.VERY versatile stuff. I use it over bare steel, then put the filler over that(bondo right on bare steel used to be done for years but this new way is better because bondo is actually porous and can allow moisture to be trapped under the bondo against the steel=rust eventually).Then use a urethane type primer surfacer (K-36 Prima)for your "sanding type primer" to finish out the body work.Prima can be tinted which down the road is nice because eventually you WILL get chips and scratches but at least the primer surfacer will be close to the same color and the chip won't show as bad.Then reduce the DP40/401 Epoxy properly to be used as a sealer (versatile= don't have to buy another product). Spray your sealer then WHATEVER system topcoat you like-DP is just about bullet proof as a sealer.I did the above for prep on "Auntie M"(avatar) then used Gillispie for topcoat but could have used any system for final color.
Pot life for DP (once mixed)is 72 hours so you get plenty of time to do what you gotta do or if you get delayed-(oh man, just got a cup mixed ready to spray the lite guards on the Cletrac and my compresser was abducted by aliens).Once applied you have up to 7 days to get the final topcoat sprayed.Longer than that and you have to sand the surface for adhesion.So if you get it all sprayed then something delays you ,you got some time.
Good stuff,simple and don't have to buy 27 different dang products(and support supplies-reducers,catalysts,etc).Of course you'll want to get with your PPG guys so they can help with any questions. I would add that using a urethane (2 part-catalyzed) primer surfacer -K-36 Prima- is the ONLY way to go. The old lacquer style (if you can still get it ) aint worth the time or money to screw your job up with.
Jim