Unless you park it in your living room, I'll put my money on the rust - it always wins. Metalprep will get into the joints, but needs to be rinsed and dried thoroughly. Zerorust suggests pouring their primer over seams and letting flow into the joint.

Now if you want to spend some $$
http://www.musclecarrestorations.com/ecoat.html
http://www.redi-coat.com/index.html

http://www.chuckthetruck.com