Epoxy primer are the best adhesion and rust protection primers available. If you apply a coat prior to stating your body work you can feel safe that you have protected it an provided the best base for your paint job.
PPG DP-LF Epoxies
These are the best primers availible for adhesion and corosion protection. They are availible in gray, red, blue and green. They also can be used as a sealer so that you do not need to buy a seperate sealer. Check with the correct P-bulliton for proper application as a sealer. Product Bulletin P-196
These are PPG's value based epoxies. They are a good product, but sometimes require an additional coat to achieve full protection and coverage. However, they are a cheaper price. Product BulletinMP170
House of Kolor Epoxy
This epoxy is a combination of an epoxy primer and a primer surfacer. This is a great product for protection and will fill body work imperfections with great sandability.